Allow null TransmitPowerRange ctor argument for ModemActivityInfo

There are existing usages in other Android code that pass null here,
causing a crash. Instead of fixing these other places (ctor is already
annotated as NonNull), let's just be graceful here as there is already
logic to handle empty input.

Bug: 143565652
Test: it builds
Change-Id: I86544bc47ff05f19a86e211fb1af87585ea68bd5