patch 7.4.1816

Problem:    Looping over a null list throws an error.
Solution:   Skip over the for loop.

src/testdir/test_expr.vim

@@ -83,3 +83,10 @@ func Test_getreg_empty_list()
   call add(x, 'foo')
   call assert_equal(['foo'], y)
 endfunc
+
+func Test_loop_over_null_list()
+  let null_list = submatch(1, 1)
+  for i in null_list
+    call assert_true(0, 'should not get here')
+  endfor
+endfunc