Add isEmpty function to Module:Opponent

[hjpalpha]

depends on #3285

Summary

Add isEmpty function to Module:Opponent
Idea is to have an easy function for checking against this case sicne we might find it useful in several modules

How did you test this change?

not tested, new functions so can not break stuff and logic is pretty simple
did not set up testcases for opponent module since it would have to (imo) include lots more test cases and i would rather add that via a sep pr at a later time

[Rathoz]

Mind splitting into 2 PRs? The bye is completely fine for instance merge, but wanna think through the empty one a bit.

[hjpalpha]

Mind splitting into 2 PRs? The bye is completely fine for instance merge, but wanna think through the empty one a bit.

#3264

[hjpalpha]

Mind splitting into 2 PRs? The bye is completely fine for instance merge, but wanna think through the empty one a bit.

for the isEmpty one as an alternative option that might be better:

---Checks if an opponent is empty
---@param opponent standardOpponent?
---@return boolean
function Opponent.isEmpty(opponent)
	return not opponent or not opponent.type
		or (String.isEmpty(opponent.name) and String.isEmpty(opponent.template) and Table.isEmpty(opponent.players))
		or (not Opponent.isTbd(opponent) and Table.deepEquals(opponent, Opponent.blank(opponent.type)))
end

[Rathoz]

Could you add some comments (why comments) for the two cases?
Like TBD Team Opponents are not considered empty