CMR-1351: Adds check for nil gran-end in validation
CMR-1351: Adds check for nil gran-end in validation

This fixes a bug in which a nil granule end datetime could results
in a failed validation. A nil end datetime should mean the granule
has no end datetime, i.e., it never ends.

nasa/Common-Metadata-Repository,nasa/Common-Metadata-Repository,nasa/Common-Metadata-Repository,mschmele/Common-Metadata-Repository,nasa/Common-Metadata-Repository,mschmele/Common-Metadata-Repository,mschmele/Common-Metadata-Repository,nasa/Common-Metadata-Repository,nasa/Common-Metadata-Repository,mschmele/Common-Metadata-Repository