patch 8.1.0736: code for Blob not sufficiently tested

Problem:    Code for Blob not sufficiently tested.
Solution:   Add more tests.  Fix uncovered crash.  Add test_null_blob().
diff --git a/src/testdir/test_eval_stuff.vim b/src/testdir/test_eval_stuff.vim
index a1aa788..dc3cb89 100644
--- a/src/testdir/test_eval_stuff.vim
+++ b/src/testdir/test_eval_stuff.vim
@@ -63,3 +63,9 @@
   call assert_fails("let v:oldfiles=''", 'E963:')
   call assert_equal(v_o, v:oldfiles)
 endfunc
+
+func Test_for_invalid()
+  call assert_fails("for x in 99", 'E714:')
+  call assert_fails("for x in 'asdf'", 'E714:')
+  call assert_fails("for x in {'a': 9}", 'E714:')
+endfunc