BatteryMonitor: Ensure batteryHealthData has value when needed

Since batteryHealthData is annotated in the AIDL with @nullable,
ensure that the std::optional that encloses it has a value when
a field needs to be assigned to batteryHealthData.

If no batteryHealthData updates are needed, the optional remains
"valueless".

Bug: 396737015
Test: Manual
Flag: EXEMPT bugfix
Change-Id: Ica6f20656a566054d7cbaaf339813be6e0e93b78