Disable inventory if player's inventory formspec is blank

[rollerozxa]

This PR intends to make it possible for games/mods to completely disable the inventory by setting the player's inventory to an empty string.

Previously if you set the player's inventory to an empty string the inventory would still exist, and opening it will open an empty default sized formspec. You could set the formspec to an extremely small value (size[0.01,0.01]) to effectively hide it, but this is a rather awful hack that will still show it and steal controls to the formspec that is now completely non-existant, and the player will now wonder if their keyboard is broken.

To do

This PR is Ready for Review.

How to test

PR tests:

• Check that if you set your inventory to an empty string (Lua code for singleplayer: minetest.get_player_by_name("singleplayer"):set_inventory_formspec("")), pressing the inventory button will do nothing.

Test for regressions:

• Check that you can still open custom non-empty inventories.
• Check that the default inventory still appears.
• Check that client-side modding formspecs don't break by this PR. (!)

[erlehmann]

Could this affect client side formspecs?

[rollerozxa]

Hopefully not, but that should definitively need to be tested, thank you for pointing that out. This PR changes code in Game::openInventory() which I'd hope to be separate of any other formspecs (minetest.show_formspec() still works with inventory disabled, I tested that), but I did not account for client-side modding.

[sfan5]

Looks good.

[erlehmann]

@sfan5 @SmallJoker how did you test the client-side formspec part?

[sfan5]

I didn't and changes to Game::openInventory() obviously won't affect it.

[SmallJoker]

@erlehmann Do you suggest to move the string.empty() check below the CSM callback? The inventory key is indeed pressed, but this callback is called when opening the inventory. If there's nothing to open, no callback is executed. What you might be talking about is a key press callback which yet does not exist.

[rollerozxa]

@erlehmann Do you suggest to move the string.empty() check below the CSM callback? The inventory key is indeed pressed, but this callback is called when opening the inventory. If there's nothing to open, no callback is executed. What you might be talking about is a key press callback which yet does not exist.

If this means CSMs adding functionality on inventory event callbacks are broken I definitively need to fix that. CSMs should be able to override a game/mod's ability to disable the inventory with it's own functionality, and I do not want to break that with this PR. I moved the .empty() check because I thought it was best to put it as high up as possible as then as little code as necessary is executed, but wasn't under the impression it would affect anything.