patch 8.0.0351: no test for concatenating an empty string

Problem:    No test for concatenating an empty string that results from out of
            bounds indexing.
Solution:   Add a simple test.

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    351,
 /**/
     350,
 /**/