patch 8.0.1422: no fallback to underline when undercurl is not set

Problem:    No fallback to underline when undercurl is not set. (Ben Jackson)
Solution:   Check for the value to be empty instead of NULL. (closes #2424)

src/version.c

@@ -771,6 +771,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1422,
 /**/
     1421,
 /**/