Drinfeld Modules: Default to zero endomorphism in .hom and avoid inversion of zero endomorphism

[S17A05]

This PR fixes #38953 and is inspired by the error raised in the issue:

• The main fix is to avoid sampling the zero element of the base field in the test of __invert__, so that phi.hom(a) actually returns an invertible morphism.
• However, the error raised in the issue named above was slightly confusing, as the call of phi.hom(K.zero()) does not return any homomorphism: Since the codomain is not specified, it tries to find a codomain via the .velu-method, but this only works if the given element defines an isogeny, which K.zero() does not. Instead of raising an error for this special case in .hom, it seems more robust to default to the zero endomorphism of self instead.
• Also added the mention of isog in the .velu-method.

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[xcaruso]

Looks good to me. Thanks.