patch 8.2.2782: Vim9: blob operations not fully tested

Problem:    Vim9: blob operations not fully tested.
Solution:   Make more blob tests run in Vim9 script.  Fix filter().  Make
            insert() give an error for a null blob, like add().
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index bf15a41..bd84a6c 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -719,6 +719,16 @@
   endfor
   res->assert_equal(6)
 
+  var lines =<< trim END
+      insert(test_null_list(), 123)
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1130:', 1)
+
+  lines =<< trim END
+      insert(test_null_blob(), 123)
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1131:', 1)
+
   assert_equal([1, 2, 3], insert([2, 3], 1))
   assert_equal([1, 2, 3], insert([2, 3], s:number_one))
   assert_equal([1, 2, 3], insert([1, 2], 3, 2))