Use map contains checks for gui textures

[Archez]

Indexing against a map for a key that doesn't exist will insert a default object constructed to that map.
The gui texture map was being indexed on lookup for texture IDs, and the texture 0 is generally the main renderer framebuffer texture. This can lead to that texture being displayed into ImGui unexpectedly.

Example of non-existent key returning framebuffer:

This adds a proper contains() check before indexing against the map to avoid inserting a default value into the map, and will return nullptr or size 0 in the following functions.