ACCUMULO-2748 added logic to handle null keys for MockTableOperations.deleteRow

Signed-off-by: Josh Elser <elserj@apache.org>
Merge branch '1.5.2-SNAPSHOT' into 1.6.0-SNAPSHOT

Conflicts:
	core/src/main/java/org/apache/accumulo/core/client/mock/MockTableOperations.java
	core/src/test/java/org/apache/accumulo/core/client/mock/MockTableOperationsTest.java
ACCUMULO-2749 make the DefaultKeySizeConstraint a bit more efficient
Merge branch '1.6.0-SNAPSHOT' of https://git-wip-us.apache.org/repos/asf/accumulo into 1.6.0-SNAPSHOT