Don't generate a bare "$" from an empty export name

[Nishuuzz]

wasm2wat --generate-names produces output it can't read back when a module has an export with an empty name.

(module (func $f) (export "" (func $f)))

Disassembled with --generate-names:

(func $ (type $t0))
(export "" (func $))

and assembling that back gives error: empty identifier.

Generated names are derived from import and export names, built as "$" followed by the name, so an empty export name leaves just the sigil. This skips an empty name instead, and the item keeps whatever name the rest of the pass gives it — another of its export names if it has one, otherwise the index-based name it would have had anyway.

Empty export names are legal and do turn up: test/wasm2c/export-names.txt already has one, which is where I hit this.

Imports go through the same helper but can't reach it, because the name there is always module_name + "." + field_name and so is never empty. I put the check in the shared helper anyway rather than in the export path, since it's guarding a property of the generated identifier rather than anything specific to exports.

How I found it

Same round-trip sweep as #2830, extended to the writer variants rather than just the default one: for every module in test/, disassemble with --fold-exprs, --inline-exports, --inline-imports, --generate-names and --no-debug-names in turn, then assemble the result again. --generate-names was 1105 of 1106, and this was the one failure. It is 1106 now.

Testing

test/roundtrip/generate-empty-export-name.txt covers a func whose only export name is empty, alongside one with a usable export name so both paths are visible in the output. It fails without the change with the empty identifier error above.

Note it deliberately does not pass --debug-names: with a name section present the original names survive and the generator never runs, so the test wouldn't exercise this at all.

roundtrip goes from 93 to 94, and desugar, typecheck, parse and the unit tests are unchanged.

Unrelated, while I was in there

The same sweep found --fold-exprs output failing to re-assemble for code metadata annotations and for branch hints:

test/dump/code-metadata.txt          error: unexpected token "metadata.code.test", expected an instr.
test/parse/branch-hints.txt          error: unexpected token (, expected ).

That looks like a separate problem in how annotations are placed in folded output, so I've left it out of this PR. Happy to open an issue for it if it isn't already known.