Set empty string to the property if value is null

Issue is if the received value is null, then while
preparing complete property value for MSIM the null
value is converting to "null" string. Fix the issue
by adding null check to the received value.

Change-Id: Ib7a8ea5ce5b1341df6155ec34c25a016eb4ec667
CRs-Fixed: 622695
SignalStrength: Hotfix TD-SCDMA

These checks need to be reviewed, they're making unwarranted assumptions
about signal value formats and possible tech combos.
This appears to work for all possible scenarios I was able to test,
but the "LTE checks" shouln't be necessary at all.

Change-Id: I889e17e79ca2b1b5fcaa393741e19bef9d3f4229
SignalStrength: Hotfix TD-SCDMA take 2

The previous approach worked for 3G, but failed with TD-LTE. Let's
try something different to make them both work, and use the derpy
LTE checks only if a GSM signal is present.

Change-Id: I77e3b5a1e0f99df30c32b102918af418a20077c1