Return False when we do not discover any environment variables

This modifies Dotenv.set_as_environment_variables to return False
if we have not discovered any environment variables via either
`dotenv_path` or `stream`.

The return value gets passed through to `load_dotenv`, so this can be
used to determine if `dotenv.load_dotenv` was able to set anything.

Closes #321

src/dotenv/main.py

@@ -87,6 +87,9 @@ def set_as_environment_variables(self) -> bool:
         """
         Load the current dotenv as system environment variable.
         """
+        if not self.dict():
+            return False
+
         for k, v in self.dict().items():
             if k in os.environ and not self.override:
                 continue

tests/test_main.py

@@ -259,8 +259,9 @@ def test_load_dotenv_no_file_verbose():
     logger = logging.getLogger("dotenv.main")
 
     with mock.patch.object(logger, "info") as mock_info:
-        dotenv.load_dotenv('.does_not_exist', verbose=True)
+        result = dotenv.load_dotenv('.does_not_exist', verbose=True)
 
+    assert result is False
     mock_info.assert_called_once_with("Python-dotenv could not find configuration file %s.", ".does_not_exist")