[9] RFR of 8073214: javadoc of Properties methods should specify NullPointerExceptions

[Brian Burkhalter]

Please review at your convenience.

Issue:	https://bugs.openjdk.java.net/browse/JDK-8073214
Patch:	http://cr.openjdk.java.net/~bpb/8073214/webrev.00/

Lines 316 and 342 in Properties.java are not strictly necessary but make it a little more fail-fast.

Thanks,

Brian