eval: Make sure that v:_null_dict does not crash dictwatcher*()

Ref neovim#4615

src/nvim/eval/typval.c

@@ -791,6 +791,9 @@ void tv_dict_watcher_add(dict_T *const dict, const char *const key_pattern,
                          const size_t key_pattern_len, ufunc_T *const callback)
   FUNC_ATTR_NONNULL_ARG(2, 4)
 {
+  if (dict == NULL) {
+    return;
+  }
   DictWatcher *const watcher = xmalloc(sizeof(DictWatcher));
   watcher->key_pattern = xmemdupz(key_pattern, key_pattern_len);
   watcher->key_pattern_len = key_pattern_len;

test/functional/ex_cmds/dict_notifications_spec.lua

@@ -255,6 +255,16 @@ describe('dictionary change notifications', function()
     end)
   end)
 
+  it('silently ignores adds to v:_null_dict', function()
+    nvim_command([[
+    function! g:Watcher1(dict, key, value)
+      call rpcnotify(g:channel, '1', a:key, a:value)
+    endfunction
+    ]])
+    eq(0,
+       exc_exec('call dictwatcheradd(v:_null_dict, "x", "g:Watcher1")'))
+  end)
+
   describe('errors', function()
     before_each(function()
       source([[
@@ -278,6 +288,11 @@ describe('dictionary change notifications', function()
         exc_exec('call dictwatcherdel(g:, "invalid_key", "g:Watcher2")'))
     end)
 
+    it('fails to remove watcher from v:_null_dict', function()
+      eq("Vim(call):Couldn't find a watcher matching key and callback",
+         exc_exec('call dictwatcherdel(v:_null_dict, "x", "g:Watcher2")'))
+    end)
+
     it("fails to add/remove if the callback doesn't exist", function()
       eq("Vim(call):Function g:InvalidCb doesn't exist",
         exc_exec('call dictwatcheradd(g:, "key", "g:InvalidCb")'))