eval/typval: Fix SEGV in test_alot.vim test

src/nvim/eval/typval.c

@@ -1167,8 +1167,11 @@ void tv_dict_unref(dict_T *const d)
 /// @return found item or NULL if nothing was found.
 dictitem_T *tv_dict_find(const dict_T *const d, const char *const key,
                          const ptrdiff_t len)
-  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_PURE FUNC_ATTR_WARN_UNUSED_RESULT
+  FUNC_ATTR_NONNULL_ARG(2) FUNC_ATTR_PURE FUNC_ATTR_WARN_UNUSED_RESULT
 {
+  if (d == NULL) {
+    return NULL;
+  }
   hashitem_T *const hi = (len < 0
                           ? hash_find(&d->dv_hashtab, (const char_u *)key)
                           : hash_find_len(&d->dv_hashtab, key, (size_t)len));

test/functional/eval/null_spec.lua

@@ -130,4 +130,10 @@ describe('NULL', function()
     null_list_expr_test('is not not equal to itself', 'L != L', 0, 0)
     null_list_expr_test('counts correctly', 'count([L], L)', 0, 1)
   end)
+  describe('dict', function()
+    it('does not crash when indexing NULL dict', function()
+      eq('\nE716: Key not present in Dictionary: test\nE15: Invalid expression: v:_null_dict.test',
+         redir_exec('echo v:_null_dict.test'))
+    end)
+  end)
 end)

test/unit/eval/typval_spec.lua

@@ -1568,5 +1568,12 @@ describe('typval.c', function()
         end)
       end)
     end)
+    describe('indexing', function()
+      describe('find', function()
+        itp('works with NULL dict', function()
+          eq(nil, lib.tv_dict_find(nil, '', 0))
+        end)
+      end)
+    end)
   end)
 end)