Improve merge bp compatibility with removing fields

[lL1l1]

Issue

Merge blueprints cannot set fields in the base bp to nil since lua does not allow storing table keys with a value of nil. The alternative to doing that is to set those keys to false, but not all code works well with that.

Description of the proposed changes

Fix errors/bugs I encountered when modding SACUs, so code using blueprints and related to enhancements, enhancement presets, and weapons.

Testing done on the proposed changes

• Units with enhancement presets that have a false value do not error and the enhancements work properly.
• Setting a preset to false does not error.
• Setting weapon buffs to false doesn't break it.

Additional context

An alternative could be to create a special value like "__nil" that table.merged treats as setting a field to nil.

Checklist

• Changes are annotated, including comments where useful
• Changes are documented in the changelog for the next game version

[lL1l1]

Another pain I encountered while modding is the Weapons table, which is an array and index 1 controls the AI. This solution seems okay-ish? At least I'm not using debug functions like I thought of initially, and it still works from inside a .bp file.

local oldModBp = ModBlueprints
---@param all_bps BlueprintsTable
ModBlueprints = function(all_bps)
    local origBp = all_bps.Unit['url0301']--[[@as UnitBlueprint]]
    local origWeapon = origBp.Weapon --[[@as WeaponBlueprint[] ]]
    table.insert(origWeapon, 1, { ... })

    oldModBp(all_bps)
end

[lL1l1]

Also there is a bug with reloading merge bps in that the mesh gets deleted due to how mesh bps are extracted and defaulted, ignoring __blueprints when merge = true. The bug causes the unit to not have a mesh and it errors out.

[lL1l1]

local oldModBp = ModBlueprints
---@param all_bps BlueprintsTable
ModBlueprints = function(all_bps)
    local origBp = all_bps.Unit['url0301']--[[@as UnitBlueprint]]
    local origWeapon = origBp.Weapon --[[@as WeaponBlueprint[] ]]
    table.insert(origWeapon, 1, { ... })

    oldModBp(all_bps)
end

Has to be table assignment and not table.insert or else you get infinite copies.

[lL1l1]

Actually you can't even use ModBlueprints from *.bp files since they're run with doscript and will hook ModBlueprints an additional time every time you reload. So your reloaded file has no guarantee for the execution order of ModBlueprints hooks within your own mod.

[lL1l1]

Closing in favor of #6669