fix system test

Author: sai-sunder-s

system_tests/noxfile.py

@@ -271,7 +271,8 @@ def compute_engine(session):
     session.install(*TEST_DEPENDENCIES_SYNC)
     # unset Application Default Credentials so
     # credentials are detected from environment
-    del session.virtualenv.env["GOOGLE_APPLICATION_CREDENTIALS"]
+    if "GOOGLE_APPLICATION_CREDENTIALS" in session.virtualenv.env:
+        del session.virtualenv.env["GOOGLE_APPLICATION_CREDENTIALS"]
     session.install(LIBRARY_DIR)
     default(
         session,