conftest.py: fix linter error

We're getting the following linter error:

tests/integration/conftest.py:468: error: Argument "resources" to
"refresh" of "Application" has incompatible type "dict[str, None]";
expected "dict[str, str] | None"  [arg-type]

We'll try to avoid it by replacing the None value with an empty string.

tests/integration/conftest.py

@@ -30,7 +30,7 @@
 log = logging.getLogger(__name__)
 TEST_DATA = Path(__file__).parent / "data"
 DEFAULT_SNAP_INSTALLATION = TEST_DATA / "default-snap-installation.tar.gz"
-DEFAULT_RESOURCES = {"snap-installation": None}
+DEFAULT_RESOURCES = {"snap-installation": ""}
 
 
 def pytest_addoption(parser: pytest.Parser):