Fix diagonal matrix construction from base ring elements that have _matrix_ methods

[mkoeppe]

The matrix constructors are designed to accept a scalar as the entries and then construct a diagonal matrix; special case: constructing the zero matrix from 0.

However, when the scalar is a base ring element that happens to have a _matrix_ method, that is tried first, leading to the errors reported in #38221.
Here we change it so that if the base ring (or matrix space) has been specified already and a scalar from that base ring is passed, that takes precedence over using a _matrix_ method. (Coercion is not attempted.)

For the special case of zero_matrix and MatrixSpace.zero_matrix, we now use entries=None, which is the fastest and most robust path in MatrixArgs.entries_type to designate a zero matrix.

Fixes #38221

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[tscrim]

LGTM.

[mkoeppe]

Thank you!