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/testdir/test_expr.vim

@@ -473,3 +473,8 @@ func Test_setmatches()
   call setmatches(set)
   call assert_equal(exp, getmatches())
 endfunc
+
+func Test_empty_concatenate()
+  call assert_equal('b', 'a'[4:0] . 'b')
+  call assert_equal('b', 'b' . 'a'[4:0])
+endfunc

src/version.c

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