Also skip null or empty fields in LfLexEntry
Also skip null or empty fields in LfLexEntry

Just like we did with LfSense objects in Mongo serialization, do the
same with LfLexEntry objects. This should result in cleaner, smaller
Mongo database records.

ermshiperete/LfMerge,ermshiperete/LfMerge,sillsdev/LfMerge,sillsdev/LfMerge,sillsdev/LfMerge,ermshiperete/LfMerge