latex: typeset proofs with optional name correctly
latex: typeset proofs with optional name correctly

Previously, we had `Proof.` if no name was given and `Some name.` if the
name was given. Now, we have `Proof (Some name).` if the name is given.

Lodifice/mfnf-pdf-export,Lodifice/mfnf-pdf-export,Lodifice/mfnf-pdf-export