[ParamTanna]

Closes #1204.

FunctionMetadata.fields() crashes with TypeError: 'NoneType' object is not iterable while building column completions for certain functions. The reporter hit it on a variadic function, and a PostgreSQL argument can arrive with no name, so arg_names (and/or arg_types) is None while arg_modes is set. The zip(self.arg_names, self.arg_types, self.arg_modes) at the bottom of fields() then fails.

args(), right above fields(), already guards this with if not self.arg_names: return []. fields() only guarded arg_modes, so it missed the case. This extends the existing early-return condition to also cover arg_names/arg_types being falsy, falling back to the function-name output column, which is what the no-output-parameters branch already does.

Reproducer without a database:

from pgcli.packages.parseutils.meta import FunctionMetadata
FunctionMetadata("public", "labels", None, ["text[]"], ["v"], "hstore", False, False, False, False, None).fields()
# before: TypeError: 'NoneType' object is not iterable
# after:  [ColumnMetadata(name='labels', datatype='hstore', ...)]

Added a regression test in tests/parseutils/test_function_metadata.py, and the existing parseutils tests still pass. Happy to add a changelog entry if you would like one.