ext/standard/string.c: use standard wording for ValueError in str_pad()

Author: Girgias

ext/standard/string.c

@@ -5802,7 +5802,7 @@ PHP_FUNCTION(str_pad)
 	}
 
 	if (pad_str_len == 0) {
-		zend_argument_value_error(3, "must be a non-empty string");
+		zend_argument_cannot_be_empty_error(3);
 		RETURN_THROWS();
 	}
 

ext/standard/tests/strings/str_pad.phpt

@@ -302,5 +302,5 @@ string(16) "\t\variation\t\t"
 #### error conditions ####
 
 --- empty padding string ---
-str_pad(): Argument #3 ($pad_string) must be a non-empty string
+str_pad(): Argument #3 ($pad_string) cannot be empty
 str_pad(): Argument #4 ($pad_type) must be STR_PAD_LEFT, STR_PAD_RIGHT, or STR_PAD_BOTH