Auto delete MetaData when = 0

[Lejo1]

We still automatic delete a key of a metadata or mod_storage data if it is = ""
This PR also auto deletes if it is = 0
As we basically also return 0 for no value this makes no compatibility problems
This saves memory when mods for example remove a player and set the var to default = 0

How to test

local storage = minetest.get_mod_storage()

storage:set_string("placeholder", "make sure file is not null")

storage:set_int("int", 100)
storage:set_int("int", 0)
storage:set_float("float", 100.1)
storage:set_float("float", 0)

storage:set_float("float2", 100.1)
storage:set_float("float2", 0.0)

storage:set_string("string", "stringggggggggggg")
storage:set_string("string", "")

All the entries will disappear without the PR float, float2 and int will stay = 0

[SmallJoker]

The only issue I can see is when you mix MetaRef:get() and MetaRef:set_int(). Latter will write "0", but former will return nil. In terms of storage optimization this is totally fine.

[Lejo1]

In my case it’s a money mod. Where I only use set_int and get_int.
When creating a function to fully reset a player including all data of all
mods I came up to this.

[Moth-Tolias]

what's the status on this?

[SmallJoker]

LGTM