refs #2450 Null value checks has been added to network merge.  This fixes NPE for keys without values.

(cherry picked from commit e28cbbc14ae242fcfbe83839051ecb3cdecaa57b)
refs #2450 Additional null checks have been added to Network Merge.

(cherry picked from commit f5cda42ef930744c9e7e41c23d06a23486664c46)
refs #2450 Invalid table values have been removed when merging networks.

(cherry picked from commit 53f125efbb94ce1d88616d7ec5fb9279865eb213)