Fix 5483: Make various calls safer from NPE and ensure searchlight target on board

[Sleet01]

Fixes an issue where the firing phase would throw an NPE if:

1. Planetary Conditions are lower light level than daylight,
2. Searchlights are on,
3. A unit that has yet to deploy, or has fled the board this turn, is at the top of the entities list

Root cause is twofold:
A) automatically selecting an off-board unit while checking to see if the selected friendly unit could use its Searchlight;
B) trying to get the hex location of units that don't have coords, in order to calculate if they are in the selected unit's arc.

This fix adds safety checks around these calls / tests to avoid this in the future, but per our Discord conversation, we
should definitely consider a non-null Coord value to represent "not on any board", especially if we will be adding more
simultaneous maps in the future.

Testing:

• Tested fix in 0.49.19.1 MM to verify handling
• Tested fix applied to 0.49.20 code via MHQ
• Ran all three projects' unit tests

Close #5483