java.util.Objects.isNull is a Java 8+ API

[chingor13]

Our libraries, though compiled on Java 8 will need to support the Java 7 API for a bit longer.

https://github.com/googleapis/java-servicedirectory/pull/244/checks?check_run_id=1485724473

[miraleung]

Thanks, fixed in #583, which went into release 0.0.8.