fix: hasSingleUnnamedParam incorrectly matching named parameters

[joelonsql]

The hasSingleUnnamedParam function was only checking the parameter type but not whether the parameter actually had no name.

This is just a minor bug, since it only cause a less precise error message than if adding the ppName == mempty check so functions with named parameters don't incorrectly match the single-param fallback.

create or replace function test.named_json_param(data json) returns json as $$
  select data;
$$ language sql;

--        it "rejects json body when single param has a name" $
--          post "/rpc/named_json_param"
--              [json|{"A": 1, "B": 2, "C": 3}|]

Eror before Fix:

  {
    "code": "42883",
    "message": "function test.named_json_param() does not exist",
    "details": null,
    "hint": "No function matches the given name and argument types..."
  }

Error after fix:

  {
    "code": "PGRST202",
    "message": "Could not find the function test.named_json_param(A, B, C) in the schema cache",
    "details": "Searched for the function test.named_json_param with parameters A, B, C or with a single unnamed json/jsonb parameter, but no matches were found in the schema cache.",
    "hint": null
  }

[steve-chavez]

@joelonsql Nice DX improvement 👍

(Edit: CI failure for commit message should go away once I squash and merge)