RFR 8206962: jlink --release-info=del throws NPE if no keys are specified

[Sundararajan Athijegannathan]

Please review.

Bug: https://bugs.openjdk.java.net/browse/JDK-8206962
Webrev: https://cr.openjdk.java.net/~sundar/8206962/webrev.00/

Thanks,
-Sundar