Fix bug where converters.py chokes...
Fix bug where converters.py chokes...

...on None nextMatePosition

Issue #727

diekhans/ga4gh-server,saupchurch/server,diekhans/ga4gh-server,srblum/server,saupchurch/server,dcolligan/server,ohsu-computational-biology/server,david4096/ga4gh-server,macieksmuga/server,ohsu-computational-biology/server,macieksmuga/server,diekhans/ga4gh-server,ga4gh/server,srblum/server,david4096/ga4gh-server,macieksmuga/server,david4096/ga4gh-server,srblum/server,dcolligan/server,ohsu-computational-biology/server,saupchurch/server,dcolligan/server,ga4gh/server,ga4gh/server