Fix ims_reasoninfo_mapping_string_array not working problem

If config ims_reasoninfo_mapping_string_array like "332||510", extra
message will map to "" instead of null.
But if there is no extra message reported, it will be null instead of "".
It causes compare fail.

Bug: 119804849
Test: Field test pass
Change-Id: Ie05f6cc55367a60e7c997a8d6e1337c0eeb16aa2
Merged-In: I042a433940d6a07d48576431445b9357ecb48ff3