hg: jdk/sandbox: 8221924: get(null) on single-entry unmodifiable Map returns null instead of throwing NPE

[chris.hegarty at oracle.com]

Changeset: 805584336738
Author:    smarks
Date:      2019-04-09 09:49 -0700
URL:       http://hg.openjdk.java.net/jdk/sandbox/rev/805584336738

8221924: get(null) on single-entry unmodifiable Map returns null instead of throwing NPE
Reviewed-by: redestad, lancea

! src/java.base/share/classes/java/util/ImmutableCollections.java
! test/jdk/java/util/Map/MapFactories.java