Remove panic from code gen of an enum's FromReflect derive.

[andriyDev]

Objective

• I noticed this while reviewing fix: Handle<T>::from_reflect incorrectly instantiating regardless of T type #24048. Our current FromReflect derive macro adds a panic for cases when a PartialReflect-ed enum includes a variant we weren't expecting. This can happen for a few reasons, e.g., a variant of the enum has been removed. In any case, crashing the program is not the correct answer, since we already have an appropriate response to a failed conversion: just return None!

Solution

• Return None instead of panicking.

I also checked if there are any other panics, and it looks like none in the codegen. The only remaining panics have to do with some auto registration file stuff, which seems appropriate to panic on.

Testing

• Added a unit test.

[makspll]

Sounds valid to me!