core: str.h - s/NULL/0/

Replace NULL with 0 (saves an extra #include).

str.h

@@ -93,7 +93,7 @@ typedef struct _str str;
  * \code str var = STR_NULL; \endcode
  * @sa STR_STATIC_INIT
  */
-#define STR_NULL {NULL, 0}
+#define STR_NULL {0, 0}
 
 /** Formats ::str string for use in printf-like functions.
  * This is a macro that prepares a ::str string for use in functions which