Fix type error

cloud-build.py:173: error: Item "None" of "Optional[IO[bytes]]" has no
attribute "read"

cloud-build.py

@@ -170,6 +170,7 @@ class CB:
                 rc = p.wait()
                 maybe_fail(string, rc)
                 with open(stdout_to_file, 'w') as f:
+                    if p.stdout:
                         f.write(p.stdout.read().decode())
             else:
                 rc = subprocess.call(cmd)