fix: don't add empty quota project  (#560)

tests/test__default.py

diff --git a/tests/test__default.py b/tests/test__default.py
index 0665efa..55a14c2 100644
--- a/tests/test__default.py
+++ b/tests/test__default.py
@@ -165,6 +165,15 @@
     assert credentials.scopes == ["https://www.google.com/calendar/feeds"]
 
 
+def test_load_credentials_from_file_service_account_with_quota_project():
+    credentials, project_id = _default.load_credentials_from_file(
+        SERVICE_ACCOUNT_FILE, quota_project_id="project-foo"
+    )
+    assert isinstance(credentials, service_account.Credentials)
+    assert project_id == SERVICE_ACCOUNT_FILE_DATA["project_id"]
+    assert credentials.quota_project_id == "project-foo"
+
+
 def test_load_credentials_from_file_service_account_bad_format(tmpdir):
     filename = tmpdir.join("serivce_account_bad.json")
     filename.write(json.dumps({"type": "service_account"}))
@@ -470,6 +479,18 @@
     return_value=(MOCK_CREDENTIALS, mock.sentinel.project_id),
     autospec=True,
 )
+def test_default_quota_project(with_quota_project):
+    credentials, project_id = _default.default(quota_project_id="project-foo")
+
+    MOCK_CREDENTIALS.with_quota_project.assert_called_once_with("project-foo")
+    assert project_id == mock.sentinel.project_id
+
+
+@mock.patch(
+    "google.auth._default._get_explicit_environ_credentials",
+    return_value=(MOCK_CREDENTIALS, mock.sentinel.project_id),
+    autospec=True,
+)
 def test_default_no_app_engine_compute_engine_module(unused_get):
     """
     google.auth.compute_engine and google.auth.app_engine are both optional