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.
diff --git a/src/option.c b/src/option.c
index 717812c..1b581ed 100644
--- a/src/option.c
+++ b/src/option.c
@@ -1990,6 +1990,8 @@
 
     // Set the new value.
     *(char_u **)(varp) = newval;
+    if (newval == NULL)
+	*(char_u **)(varp) = empty_option;
 
 #if defined(FEAT_EVAL)
     if (!starting
diff --git a/src/version.c b/src/version.c
index a6451cb..c59a3a1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1311,
+/**/
     1310,
 /**/
     1309,