check if @rake_patterns is defined
check if @rake_patterns is defined

This removes the following warning.
```
railties/lib/rails/test_unit/minitest_plugin.rb:45: warning: instance variable @rake_patterns not initialize
```

sealocal/rails,ledestin/rails,bogdanvlviv/rails,yahonda/rails,vipulnsward/rails,kenta-s/rails,coreyward/rails,MSP-Greg/rails,xlymian/rails,yahonda/rails,esparta/rails,mtsmfm/rails,hanystudy/rails,alecspopa/rails,vassilevsky/rails,kmayer/rails,prathamesh-sonpatki/rails,Vasfed/rails,rails/rails,starknx/rails,gauravtiwari/rails,mechanicles/rails,felipecvo/rails,bogdanvlviv/rails,kamipo/rails,richseviora/rails,illacceptanything/illacceptanything,palkan/rails,koic/rails,jeremy/rails,kenta-s/rails,voray/rails,stefanmb/rails,untidy-hair/rails,Envek/rails,rafaelfranca/omg-rails,samphilipd/rails,utilum/rails,schuetzm/rails,palkan/rails,marklocklear/rails,voray/rails,illacceptanything/illacceptanything,georgeclaghorn/rails,schuetzm/rails,ledestin/rails,mathieujobin/reduced-rails-for-travis,illacceptanything/illacceptanything,yasslab/railsguides.jp,pvalena/rails,hanystudy/rails,schuetzm/rails,yhirano55/rails,mtsmfm/rails,gfvcastro/rails,yalab/rails,odedniv/rails,vipulnsward/rails,MSP-Greg/rails,mohitnatoo/rails,rafaelfranca/omg-rails,maicher/rails,illacceptanything/illacceptanything,richseviora/rails,betesh/rails,tjschuck/rails,palkan/rails,brchristian/rails,arjes/rails,gavingmiller/rails,starknx/rails,Stellenticket/rails,BlakeWilliams/rails,kddeisz/rails,lucasmazza/rails,aditya-kapoor/rails,stefanmb/rails,koic/rails,marklocklear/rails,Edouard-chin/rails,elfassy/rails,Edouard-chin/rails,matrinox/rails,gavingmiller/rails,MSP-Greg/rails,esparta/rails,jeremy/rails,maicher/rails,mohitnatoo/rails,elfassy/rails,yalab/rails,utilum/rails,baerjam/rails,Envek/rails,yahonda/rails,kachick/rails,sergey-alekseev/rails,lcreid/rails,deraru/rails,bogdanvlviv/rails,kmcphillips/rails,illacceptanything/illacceptanything,printercu/rails,bogdanvlviv/rails,tjschuck/rails,tgxworld/rails,stefanmb/rails,voray/rails,kirs/rails-1,travisofthenorth/rails,rossta/rails,iainbeeston/rails,tjschuck/rails,shioyama/rails,baerjam/rails,bradleypriest/rails,kmayer/rails,kenta-s/rails,arunagw/rails,illacceptanything/illacceptanything,schuetzm/rails,illacceptanything/illacceptanything,lcreid/rails,pschambacher/rails,shioyama/rails,mathieujobin/reduced-rails-for-travis,tijwelch/rails,yhirano55/rails,jeremy/rails,illacceptanything/illacceptanything,rails/rails,illacceptanything/illacceptanything,brchristian/rails,aditya-kapoor/rails,bradleypriest/rails,bolek/rails,gauravtiwari/rails,aditya-kapoor/rails,yhirano55/rails,mtsmfm/railstest,yalab/rails,tgxworld/rails,yawboakye/rails,arjes/rails,eileencodes/rails,mechanicles/rails,kmayer/rails,ttanimichi/rails,kddeisz/rails,illacceptanything/illacceptanything,travisofthenorth/rails,pvalena/rails,tgxworld/rails,Erol/rails,richseviora/rails,vipulnsward/rails,odedniv/rails,Erol/rails,illacceptanything/illacceptanything,samphilipd/rails,notapatch/rails,illacceptanything/illacceptanything,joonyou/rails,kddeisz/rails,flanger001/rails,yahonda/rails,rbhitchcock/rails,untidy-hair/rails,fabianoleittes/rails,esparta/rails,rafaelfranca/omg-rails,mijoharas/rails,notapatch/rails,Edouard-chin/rails,travisofthenorth/rails,marklocklear/rails,EmmaB/rails-1,rbhitchcock/rails,georgeclaghorn/rails,Envek/rails,baerjam/rails,yalab/rails,eileencodes/rails,yasslab/railsguides.jp,coreyward/rails,travisofthenorth/rails,pschambacher/rails,kamipo/rails,gfvcastro/rails,vassilevsky/rails,Spin42/rails,Stellenticket/rails,gavingmiller/rails,assain/rails,riseshia/railsguides.kr,tgxworld/rails,sergey-alekseev/rails,assain/rails,pvalena/rails,alecspopa/rails,mechanicles/rails,ledestin/rails,flanger001/rails,prathamesh-sonpatki/rails,sealocal/rails,joonyou/rails,lcreid/rails,kachick/rails,kmcphillips/rails,arjes/rails,fabianoleittes/rails,ttanimichi/rails,tjschuck/rails,kmcphillips/rails,vassilevsky/rails,ngpestelos/rails,Sen-Zhang/rails,gauravtiwari/rails,odedniv/rails,kaspth/rails,betesh/rails,deraru/rails,mathieujobin/reduced-rails-for-travis,Erol/rails,Erol/rails,printercu/rails,Sen-Zhang/rails,iainbeeston/rails,samphilipd/rails,untidy-hair/rails,MichaelSp/rails,maicher/rails,gcourtemanche/rails,ttanimichi/rails,sergey-alekseev/rails,bolek/rails,BlakeWilliams/rails,betesh/rails,repinel/rails,flanger001/rails,EmmaB/rails-1,mechanicles/rails,xlymian/rails,arunagw/rails,mtsmfm/railstest,matrinox/rails,rossta/rails,shioyama/rails,rails/rails,gcourtemanche/rails,kirs/rails-1,amoody2108/TechForJustice,assain/rails,untidy-hair/rails,hanystudy/rails,felipecvo/rails,kmcphillips/rails,pvalena/rails,illacceptanything/illacceptanything,bradleypriest/rails,kachick/rails,eileencodes/rails,palkan/rails,rails/rails,Stellenticket/rails,aditya-kapoor/rails,esparta/rails,gcourtemanche/rails,bolek/rails,kamipo/rails,rossta/rails,repinel/rails,arunagw/rails,iainbeeston/rails,Vasfed/rails,yawboakye/rails,yhirano55/rails,joonyou/rails,elfassy/rails,Edouard-chin/rails,ngpestelos/rails,EmmaB/rails-1,fabianoleittes/rails,koic/rails,BlakeWilliams/rails,sealocal/rails,mijoharas/rails,eileencodes/rails,printercu/rails,felipecvo/rails,Vasfed/rails,BlakeWilliams/rails,riseshia/rai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