patch 8.2.1924: Vim9: crash when indexing dict with NULL key

Problem:    Vim9: crash when indexing dict with NULL key.
Solution:   Use empty string instead of NULL. (closes #7229)  Make error
            message more useful for empty string.
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 568c3bf..8f691e6 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -1917,6 +1917,7 @@
   CheckDefExecFailure(['var x: dict<string> = #{a: "x", b: 134}'], 'E1012:', 1)
 
   CheckDefFailure(['var x = ({'], 'E723:', 2)
+  CheckDefExecFailure(['{}[getftype("")]'], 'E716: Key not present in Dictionary: ""', 1)
 enddef
 
 def Test_expr7_dict_vim9script()