patch 8.2.1952: Vim9: crash when using a NULL dict key

Problem:    Vim9: crash when using a NULL dict key.
Solution:   Use a NULL dict key like an empty string. (closes #7249)
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 656718c..2d741e4 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -1902,6 +1902,8 @@
   var dictdict: dict<dict<string>> = #{one: #{a: 'text'}, two: #{}}
   dictdict = #{one: #{}, two: #{a: 'text'}}
   dictdict = #{one: #{}, two: #{}}
+
+  assert_equal({'': 0}, {matchstr('string', 'wont match'): 0})
  
   CheckDefFailure(["var x = #{a:8}"], 'E1069:', 1)
   CheckDefFailure(["var x = #{a : 8}"], 'E1068:', 1)