Fix Issue Where SignalStrengthGsm is null

Need to check for isEmpty() not null when determining
if SignalStrength needs a fixup.

Bug: 135492584
Test: atest RILTest#testFixupSignalStrength10
Merged-In: If140fe7a3521987ef2ab1db8d0a0326ac72d89cf
Change-Id: If140fe7a3521987ef2ab1db8d0a0326ac72d89cf
(cherry picked from commit 73c30705)