[RFE] Have Board return a generic "Off-Board Hex" hex type when `.getHex()` called with an off-board position

[Sleet01]

Enhancement Type *

New Feature

Brief Description of Enhancement *

Problem:

We keep finding NPEs when various checks require a Hex instance but can happen to units that are off-board.
Board.getHex() returns a null value if the position it receives is not on the game board, but the code is full of chained calls that expect a Hex instance so they can query it for terrain or level information.

Proposed solution:

1. Create an "off-board Hex" subclass (or flag field in Hex) so that Board can return a valid Hex instance when a position is off-board, which will return various default values for terrain queries. This is similar to the way Targetable works for hexes.
2. Find all instances where we check for hex == null and remove them, or replace them with a check for isOffboard().
3. [Optional] also create isOnboard() to avoid lots of !hex.isOffboard() calls.

We should be able to replace a number of null checks, cleaning up the code, while also laying a foundation for future off-board functionality.

Use Cases or Rationale

1. Handling some off-board artillery attacks (counter-battery fire)
2. Handling SPAs that refer to specific Terrain types but which may be present in off-board units.
3. [Future] possibly allowing integration of low-altitude and ground maps for e.g. AA ground units and bombing on the Low-Altitude map.

Attach Files

No response

MegaMek Suite Version *

0.50.03

Final Verification

• I confirm this request hasn't already been submitted (checked the tracker)
• I've discussed or asked about this enhancement on MegaMek Discord
• I’m opening this on the correct repo (MegaMek, MegaMekLab, or MekHQ)

[HoneySkull]

This is a good idea. I recommend it.