fix: retrieve the zero-argument function overload when args is empty

[hsusul]

What kind of change does this PR introduce?

Bug fix.

Why?

functions.retrieve({ schema, name, args: [] }) is documented by the existing tests as selecting the zero-argument overload. FUNCTIONS_SQL skipped the proargtypes predicate whenever args was empty, so overloaded names returned data[0] instead of the no-arg signature.

functions.create() looks the function up the same way after CREATE FUNCTION, so creating a zero-argument overload of an existing name could also return the wrong row.

Fixes #1111.

How?

When args is provided, always filter on p.proargtypes::text. Non-empty argument lists still resolve type OIDs the same way; args: [] now compares against the empty oidvector (''), which was already present as dead code.

Checklist

• Bug fix
• Tests added
• Docs
• Breaking change

New tests

retrieve function by args filter - zero-argument overload among polymorphic functions uses the existing polymorphic_function_with_no_params_or_unnamed fixtures. Before the fix it received the text overload; after the fix it receives the zero-argument integer overload.