Update metadata.avdl
Update metadata.avdl

removal of the NULL option for ```recordCreateTime``` and ```recordUpdateTime``` instances
hjellinek/schemas,dnanexus-rnd/schemas,laserson/schemas,Jeltje/ga4gh-schemas,saupchurch/schemas,buske/schemas,lh3/schemas,lh3/schemas,laserson/schemas,sarahhunt/schemas,benedictpaten/schemas,ga4gh/schemas,diekhans/ga4gh-schemas,delagoya/schemas,pcingola/schemas,dglazer/schemas,heuermh/ga4gh-schemas,diekhans/ga4gh-schemas,kellrott/schemas,adamnovak/schemas,adamnovak/schemas,benedictpaten/schemas,kellrott/schemas,macieksmuga/schemas,sarahhunt/schemas,dcolligan/schemas,nlwashington/schemas,dnanexus-rnd/schemas,delagoya/schemas,ga4gh/ga4gh-schemas,buske/schemas,hjellinek/jeltje-schemas-sphinx,Jeltje/ga4gh-schemas,nlwashington/schemas,hjellinek/schemas