Remove ill-conceived ban on zero length json object keys.

We removed a similar ban on this in json_object recently, but the ban in
datum_to_json was left, which generate4d sprutious errors in othee json
generators, notable json_build_object.

Along the way, add an assertion that datum_to_json is not passed a null
key. All current callers comply with this rule, but the assertion will
catch any possible future misbehaviour.

src/backend/utils/adt/json.c

@@ -1342,6 +1342,9 @@ datum_to_json(Datum val, bool is_null, StringInfo result,
 	bool		numeric_error;
 	JsonLexContext dummy_lex;
 
+	/* callers are expected to ensure that null keys are not passed in */
+	Assert( ! (key_scalar && is_null));
+
 	if (is_null)
 	{
 		appendStringInfoString(result, "null");
@@ -1487,10 +1490,6 @@ datum_to_json(Datum val, bool is_null, StringInfo result,
 			break;
 		default:
 			outputstr = OidOutputFunctionCall(outfuncoid, val);
-			if (key_scalar && *outputstr == '\0')
-				ereport(ERROR,
-						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-						 errmsg("key value must not be empty")));
 			escape_json(result, outputstr);
 			pfree(outputstr);
 			break;