Fix #6507: autosummary: crashes without no autosummary_generate setting

tests/test_ext_autosummary.py

@@ -302,3 +302,11 @@ def test_generate_autosummary_docs_property(app):
                        ".. currentmodule:: target.methods\n"
                        "\n"
                        ".. autoproperty:: Base.prop")
+
+
+@pytest.mark.sphinx('dummy', testroot='ext-autosummary',
+                    confoverrides={'autosummary_generate': []})
+def test_empty_autosummary_generate(app, status, warning):
+    app.build()
+    assert ("WARNING: autosummary: stub file not found 'autosummary_importfail'"
+            in warning.getvalue())