vtgatev3_test.py: Fix teardown exception when keyspace_env is None.
vtgatev3_test.py: Fix teardown exception when keyspace_env is None.

This happens when the test exits early because a previous test was not cleaned up correctly.
enisoc/vitess,mattharden/vitess,pivanof/vitess,mahak/vitess,vitessio/vitess,mapbased/vitess,rnavarro/vitess,aaijazi/vitess,mattharden/vitess,applift/vitess,applift/vitess,alainjobart/vitess,dumbunny/vitess,AndyDiamondstein/vitess,erzel/vitess,pivanof/vitess,tirsen/vitess,dcadevil/vitess,mapbased/vitess,applift/vitess,mapbased/vitess,AndyDiamondstein/vitess,AndyDiamondstein/vitess,guokeno0/vitess,erzel/vitess,tirsen/vitess,HubSpot/vitess,pivanof/vitess,mapbased/vitess,tinyspeck/vitess,vitessio/vitess,enisoc/vitess,tirsen/vitess,vitessio/vitess,applift/vitess,rnavarro/vitess,tinyspeck/vitess,dcadevil/vitess,rnavarro/vitess,mattharden/vitess,tirsen/vitess,rnavarro/vitess,alainjobart/vitess,tirsen/vitess,guokeno0/vitess,applift/vitess,enisoc/vitess,mahak/vitess,davygeek/vitess,pivanof/vitess,aaijazi/vitess,enisoc/vitess,mattharden/vitess,rnavarro/vitess,HubSpot/vitess,dumbunny/vitess,tirsen/vitess,dumbunny/vitess,applift/vitess,dcadevil/vitess,mahak/vitess,HubSpot/vitess,HubSpot/vitess,rnavarro/vitess,sougou/vitess,dumbunny/vitess,dumbunny/vitess,alainjobart/vitess,mahak/vitess,vitessio/vitess,mattharden/vitess,AndyDiamondstein/vitess,erzel/vitess,dcadevil/vitess,alainjobart/vitess,aaijazi/vitess,mapbased/vitess,pivanof/vitess,rnavarro/vitess,rnavarro/vitess,davygeek/vitess,applift/vitess,aaijazi/vitess,guokeno0/vitess,sougou/vitess,davygeek/vitess,HubSpot/vitess,aaijazi/vitess,AndyDiamondstein/vitess,pivanof/vitess,vitessio/vitess,tirsen/vitess,guokeno0/vitess,sougou/vitess,pivanof/vitess,applift/vitess,applift/vitess,guokeno0/vitess,erzel/vitess,mapbased/vitess,pivanof/vitess,alainjobart/vitess,mattharden/vitess,dcadevil/vitess,tinyspeck/vitess,HubSpot/vitess,davygeek/vitess,erzel/vitess,alainjobart/vitess,HubSpot/vitess,guokeno0/vitess,sougou/vitess,davygeek/vitess,guokeno0/vitess,tinyspeck/vitess,mattharden/vitess,AndyDiamondstein/vitess,applift/vitess,aaijazi/vitess,tinyspeck/vitess,tirsen/vitess,mahak/vitess,alainjobart/vitess,tinyspeck/vitess,erzel/vitess,sougou/vitess,mapbased/vitess,mahak/vitess,guokeno0/vitess,aaijazi/vitess,dumbunny/vitess,davygeek/vitess,dumbunny/vitess,erzel/vitess,enisoc/vitess,vitessio/vitess,dumbunny/vitess,dumbunny/vitess,sougou/vitess,AndyDiamondstein/vitess,mapbased/vitess,sougou/vitess,alainjobart/vitess,mattharden/vitess,davygeek/vitess,mahak/vitess,AndyDiamondstein/vitess,mahak/vitess,pivanof/vitess,vitessio/vitess,HubSpot/vitess,rnavarro/vitess,mapbased/vitess,erzel/vitess,tirsen/vitess,vitessio/vitess,enisoc/vitess,erzel/vitess,mattharden/vitess,dumbunny/vitess,dcadevil/vitess,aaijazi/vitess,guokeno0/vitess,dcadevil/vitess,mattharden/vitess,erzel/vitess,enisoc/vitess,AndyDiamondstein/vitess,tinyspeck/vitess,mapbased/vitess,aaijazi/vitess,sougou/vitess