Improve guards for `table.copy` to fix blueprint reloading crash

[lL1l1]

Description of the proposed changes

Fixes in-game blueprint reloading crash caused by #6273 (specific diff) from blueprints-units.lua:311 due to spawn dummies having CommandCaps = "0". Instead of checking for nil, checks for the value to be a table.

The function could use a better annotation if possible.

Testing done on the proposed changes

The game no longer crashes when reloading blueprints during a session.

Checklist

• [x] Changes are annotated, including comments where useful - [ ] Changes are documented in the changelog for the next game version Not necessary.

[Garanas]

What about:

--- table.copy(t) returns a shallow copy of t.
---@generic T
---@param t T
---@return T
function table.copy(t)
  (...)
end

[lL1l1]

I misunderstood what a shallow copy is. I thought it would only copy 1 deep (which doesn't preserve type) but it just copies old memory references into the new indexes of the new table (which does preserve type, but makes mutating it need more care).