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.