fix(js-parser): treat a zero-JS-file repo as an empty result, not a crash

[gadievron]

A directory with zero JS-family source files made the JavaScript parser
abort the entire scan instead of returning a clean empty result.

repository_scanner.js succeeds and emits an empty file list (exit 0), but
run_typescript_analyzer hit if not files: return False, so
run_full_pipeline early-returned without results['success'], main() did
sys.exit(1), and _parse_javascript raised RuntimeError on the non-zero
exit (uncaught at the scan layer => whole scan aborts). The Python, Ruby
and Zig parsers all handle an empty repo gracefully; the JS guard was the
lone outlier promoting valid empty input to a fatal exit.

Mirror the zig graceful-empty pattern entirely within
parsers/javascript/test_pipeline.py: on zero files, write valid empty
analyzer_output.json / dataset.json / call_graph.json, record synthetic
successful stages, set self.empty_repo, and return True; run_full_pipeline
then skips the unit generator and reports success. _parse_javascript reads
units_count=0 and returns a valid empty ParseResult. parser_adapter.py and
zig/test_pipeline.py are correct as-is and left untouched.

Adds regression test tests/parsers/javascript/test_empty_repo.py
(RED before, GREEN after). Full openant-core suite: 219 passed, 22 skipped.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com