Add support for remapping ImsReasonInfo codes with no message.

There was an assumption that the KEY_IMS_REASONINFO_MAPPING_STRING_ARRAY
reason codes would always remap a pair of code and message to a new
ImsReasonInfo code.

This change relaxes that restriction to support straight remapping of
ImsReasonInfo codes for a carrier where the message is empty/null.

Test: Added unit test to cover new functionality.
Bug: 110068190
Change-Id: I07e4ce8b9f4c64a18e76737b313f270cd8c7531e