patch 8.2.4526: Vim9: cannot set variables to a null value

Problem:    Vim9: cannot set variables to a null value.
Solution:   Add null_list, null_job, etc.

src/testdir/test_vim9_assign.vim

@@ -306,12 +306,44 @@ def Test_assign_register()
 enddef
 
 def Test_reserved_name()
-  for name in ['true', 'false', 'null']
+  var more_names = ['null_job', 'null_channel']
+  if !has('job')
+    more_names = []
+  endif
+
+  for name in ['true',
+               'false',
+               'null',
+               'null_blob',
+               'null_dict',
+               'null_function',
+               'null_list',
+               'null_partial',
+               'null_string',
+               ] + more_names
     v9.CheckDefExecAndScriptFailure(['var ' .. name .. ' =  0'], 'E1034:')
     v9.CheckDefExecAndScriptFailure(['var ' .. name .. ': bool'], 'E1034:')
   endfor
 enddef
 
+def Test_null_values()
+  var lines =<< trim END
+      var b: blob = null_blob
+      var dn: dict<number> = null_dict
+      var ds: dict<string> = null_dict
+      var ln: list<number> = null_list
+      var ls: list<string> = null_list
+      var Ff: func(string): string = null_function
+      var Fp: func(number): number = null_partial
+      var s: string = null_string
+      if has('job')
+        var j: job = null_job
+        var c: channel = null_channel
+      endif
+  END
+  v9.CheckDefAndScriptSuccess(lines)
+enddef
+
 def Test_skipped_assignment()
   var lines =<< trim END
       for x in []