[AIRFLOW-694] Fix config behaviour for empty envvar

Currently, environment variable with empty value
does not overwrite the
configuration value corresponding to it.

Closes apache#2044 from sekikn/AIRFLOW-694

airflow/configuration.py

@@ -591,7 +591,7 @@ def get(self, section, key, **kwargs):
 
         # first check environment variables
         option = self._get_env_var_option(section, key)
-        if option:
+        if option is not None:
             return option
 
         # ...then the config file

tests/core.py

@@ -776,6 +776,30 @@ def test_config_throw_error_when_original_and_fallback_is_absent(self):
         configuration.set("core", "FERNET_KEY", FERNET_KEY)
         assert configuration.has_option("core", "FERNET_KEY")
 
+    def test_config_override_original_when_non_empty_envvar_is_provided(self):
+        key = "AIRFLOW__CORE__FERNET_KEY"
+        value = "some value"
+        assert key not in os.environ
+
+        os.environ[key] = value
+        FERNET_KEY = configuration.get('core', 'FERNET_KEY')
+        assert FERNET_KEY == value
+
+        # restore the envvar back to the original state
+        del os.environ[key]
+
+    def test_config_override_original_when_empty_envvar_is_provided(self):
+        key = "AIRFLOW__CORE__FERNET_KEY"
+        value = ""
+        assert key not in os.environ
+
+        os.environ[key] = value
+        FERNET_KEY = configuration.get('core', 'FERNET_KEY')
+        assert FERNET_KEY == value
+
+        # restore the envvar back to the original state
+        del os.environ[key]
+
     def test_class_with_logger_should_have_logger_with_correct_name(self):
 
         # each class should automatically receive a logger with a correct name