patch 9.0.1311: Coverity warns for using a NULL pointer

Problem:    Coverity warns for using a NULL pointer.
Solution:   Use "empty_option" instead of NULL.

src/option.c

@@ -1990,6 +1990,8 @@ do_set_option_string(
 
     // Set the new value.
     *(char_u **)(varp) = newval;
+    if (newval == NULL)
+	*(char_u **)(varp) = empty_option;
 
 #if defined(FEAT_EVAL)
     if (!starting

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1311,
 /**/
     1310,
 /**/