Fix behaviour of LazyDictWithCache wheni import fails

When apache#15330 added docker.task, it also optimized replacement of the
callable with it's result in LazyDictWithCache. LazyDictWithCache
is used by Provider's Manager to optimize access to hooks - basically
hook is only actually imported, when it is accessed. This helps with
speeding up importing of connection information

The optimization added result of running callable to _resolved
set, but it missed the case when None was returned. Previously,
when None was returned, the callable was not replaced and it
was called again. After the change - the _resolved set was
updated with the key and None was returned. But since the key
has not been replaced, next time when the same key was retrieved,
the original "callable" was returned, not the None value. So if
callable returned None, and the same key was retrieved twice, the second
time, instead of None, the dictionary returned Callable.

This PR fixes it by setting the value to dictionary even if it
was None.

Author: potiuk

airflow/providers_manager.py

@@ -109,8 +109,7 @@ def __getitem__(self, key):
             # callable itself
             value = value()
             self._resolved.add(key)
-            if value:
-                self._raw_dict.__setitem__(key, value)
+            self._raw_dict.__setitem__(key, value)
         return value
 
     def __delitem__(self, key):

tests/always/test_providers_manager.py

@@ -28,7 +28,7 @@
 from wtforms import BooleanField, Field, StringField
 
 from airflow.exceptions import AirflowOptionalProviderFeatureException
-from airflow.providers_manager import HookClassProvider, ProviderInfo, ProvidersManager
+from airflow.providers_manager import HookClassProvider, LazyDictWithCache, ProviderInfo, ProvidersManager
 
 
 class TestProviderManager:
@@ -373,3 +373,32 @@ def test_optional_feature_debug(self, mock_importlib_import_string):
             assert [
                 "Optional provider feature disabled when importing 'HookClass' from 'test_package' package"
             ] == self._caplog.messages
+
+
+@pytest.mark.parametrize(
+    "value, expected_outputs,",
+    [
+        ("a", "a"),
+        (1, 1),
+        (None, None),
+        (lambda: 0, 0),
+        (lambda: None, None),
+        (lambda: "z", "z"),
+    ],
+)
+def test_lazy_cache_dict_resolving(value, expected_outputs):
+    lazy_cache_dict = LazyDictWithCache()
+    lazy_cache_dict["key"] = value
+    assert lazy_cache_dict["key"] == expected_outputs
+    # Retrieve it again to see if it is correctly returned again
+    assert lazy_cache_dict["key"] == expected_outputs
+
+
+def test_lazy_cache_dict_raises_error():
+    def raise_method():
+        raise Exception("test")
+
+    lazy_cache_dict = LazyDictWithCache()
+    lazy_cache_dict["key"] = raise_method
+    with pytest.raises(Exception, match="test"):
+        _ = lazy_cache_dict["key"]