Fixed two tests incorrectly using StringIO

wagtail/search/tests/test_backends.py

@@ -926,7 +926,7 @@ class BackendTests(WagtailTestUtils):
         management.call_command(
             "update_index", verbosity=0, backend_name=self.backend_name, stdout=stdout
         )
-        self.assertFalse(stdout.read())
+        self.assertFalse(stdout.getvalue())
 
 
 @override_settings(

wagtail/tests/test_reference_index.py

@@ -238,7 +238,7 @@ class TestCreateOrUpdateForObject(TestCase):
             verbosity=0,
             stdout=stdout,
         )
-        self.assertFalse(stdout.read())
+        self.assertFalse(stdout.getvalue())
 
 
 class TestDescribeOnDelete(TestCase):