Proto/Michelson: annotations are non-empty

Context

Annotations are non-empty strings (A), though there are several places where we match them against the empty string, which is confusing. Let's enforce (A) with types.

(A) is not actually true, there was a case in parse_constr_annot which allowed constructing empty annotations. This is now fixed. ⚠ There is a risk of breaking existing code.

It happened when PAIR/LEFT/RIGHT was used with the special annot %@ on a value with a var annot of the form @abc.

Prerequisite for #2025 (closed).

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