Prevent NPE in TStructDescriptor when an unexpected enum is encountered by returning an empty map.
Check for null maps in enum-related TStructDescriptor methods.
Merge branch 'master' of git://github.com/kevinweil/elephant-bird into tstruct_enum_npe

Conflicts:
	core/src/main/java/com/twitter/elephantbird/thrift/TStructDescriptor.java
Merge pull request #364 from dvryaboy/tstruct_enum_npe

Prevent NPE in TStructDescriptor when an unexpected enum is encountered