Error when string.split is given empty separator (#13132)

doc/lua_api.txt

@@ -3623,7 +3623,7 @@ Helper functions
 * `math.round(x)`: Returns `x` rounded to the nearest integer.
     * At a multiple of 0.5, rounds away from zero.
 * `string.split(str, separator, include_empty, max_splits, sep_is_pattern)`
-    * `separator`: string, default: `","`
+    * `separator`: string, cannot be empty, default: `","`
     * `include_empty`: boolean, default: `false`
     * `max_splits`: number, if it's negative, splits aren't limited,
       default: `-1`