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)
diff --git a/src/version.c b/src/version.c
index 943469f..65d5905 100644
--- a/src/version.c
+++ b/src/version.c
@@ -772,6 +772,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1422,
+/**/
     1421,
 /**/
     1420,