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.
diff --git a/src/testdir/test_expr.vim b/src/testdir/test_expr.vim
index e5c918c..80cdc1b 100644
--- a/src/testdir/test_expr.vim
+++ b/src/testdir/test_expr.vim
@@ -473,3 +473,8 @@
   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
diff --git a/src/version.c b/src/version.c
index bd271cc..b51f04d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    351,
+/**/
     350,
 /**/
     349,