Fix empty admin search on Elasticsearch (backport to 5.2)

Backport of #11626 to 5.2; fixes #11600

wagtail/search/backends/elasticsearch5.py

@@ -653,8 +653,15 @@ class ElasticsearchAutocompleteQueryCompilerImpl:
         if len(fields) == 0:
             # No fields. Return a query that'll match nothing
             return {"bool": {"mustNot": {"match_all": {}}}}
-
-        return self._compile_plaintext_query(self.query, fields)
+        elif isinstance(self.query, PlainText):
+            return self._compile_plaintext_query(self.query, fields)
+        elif isinstance(self.query, MatchAll):
+            return {"match_all": {}}
+        else:
+            raise NotImplementedError(
+                "`%s` is not supported for autocomplete queries."
+                % self.query.__class__.__name__
+            )
 
 
 class Elasticsearch5AutocompleteQueryCompiler(

wagtail/search/tests/test_elasticsearch5_backend.py

@@ -70,6 +70,31 @@ class TestElasticsearch5SearchQuery(TestCase):
         }
         self.assertDictEqual(query_compiler.get_query(), expected_result)
 
+    def test_match_all_autocomplete(self):
+        # Create a query
+        query = self.autocomplete_query_compiler_class(
+            models.Book.objects.all(), MATCH_ALL
+        )
+
+        # Check it
+        expected_result = {
+            "bool": {
+                "filter": {"match": {"content_type": "searchtests.Book"}},
+                "must": {"match_all": {}},
+            }
+        }
+        self.assertDictEqual(query.get_query(), expected_result)
+
+    def test_non_supported_queries_autocomplete(self):
+        # Create a query
+        query = self.autocomplete_query_compiler_class(
+            models.Book.objects.all(), Fuzzy("Hello")
+        )
+
+        # Check it
+        with self.assertRaises(NotImplementedError):
+            query.get_query()
+
     def test_match_all(self):
         # Create a query
         query_compiler = self.query_compiler_class(models.Book.objects.all(), MATCH_ALL)