Fix ModMenu compatibility

[kikugie]

ConfigScreenFactory factory in ModMenu is allowed to be null, in which case the entry is skipped. This behavior is intentional and allows mods to have optional dependencies on config libraries.
I.e. if you return null in getModConfigScreenFactory() it means "I would like to be compatible with modmenu, but can't rn".

This PR adds this check to the mod to replicate the behavior and fix a crash when a mod follows this pattern.

[MrCrayfish]

Thanks for the info. I should have looked at the code more closely, however it seems the method docs doesn't mention this looking at it now. Should getProvidedConfigScreenFactories() be prevented from running if getModConfigScreenFactory() returns null?

[MrCrayfish]

The reason I followed non-null is because it's not documented. I have submitted an issue on Mod Menu repo (TerraformersMC/ModMenu#724) to clarify the problem. Once I get a response, I'll decide what happens.

[MrCrayfish]

Another point I bring up is that returning null on ConfigScreenFactory#create can be used to do exactly the same, since returning a null Screen will do nothing too. Regardless if you return null for the factory, it is still put into the ModMenu#configScreenFactories map.

[kikugie]

Both approaches are valid in modmenu, but one causes a crash with this mod, so iono looks worth it to me

[MrCrayfish]

Fixed in 2b18b42