Fix FunctionMetadata.fields() crash when arg_names is None

[Tatamis]

Fixes #1204.

Bug

Autocompleting a call to a table-returning or variadic function whose parameters have no names (e.g. an unnamed variadic text[] argument, as in the reported hstore/labels(variadic text[]) example) crashes the completion thread:

File ".../pgcli/pgcompleter.py", line 985, in populate_scoped_cols
    cols = func.fields()
File ".../pgcli/packages/parseutils/meta.py", line 168, in fields
    for name, typ, mode in zip(self.arg_names, self.arg_types, self.arg_modes)
Exception 'NoneType' object is not iterable

fields() only guards against a missing arg_modes (elif not self.arg_modes: ...), then zips arg_names/arg_types/arg_modes unconditionally. arg_modes being truthy doesn't guarantee arg_names is populated too — a function can have modes but no names, in which case arg_names stays None and the zip() raises before it ever gets to filter by mode.

Fix

Fall back to None placeholders for arg_names/arg_types the same way args() (a few lines up in the same class) already falls back for arg_modes, so the zip always gets iterables.

Test plan

• Added test_function_metadata_fields_with_variadic_and_no_arg_names, directly reproducing the reported case via FunctionMetadata.
• Added test_function_metadata_fields_table_mode_with_no_arg_names, checking a TABLE-mode function with unnamed columns returns sensible ColumnMetadata (not just an empty list).
• Verified both fail with the exact reported TypeError against the unpatched code.
• pytest tests/ --ignore=tests/features — 2575 passed, 1 pre-existing failure unrelated to this change (a Windows-only tempfile permission issue in an unrelated alias-map test), 1 xfailed, 1 xpassed.
• ruff check / ruff format clean.