SCHEMA-228. FormattedKeys: allow nulls in hash calculation

Formatted Keys throw IndexOutOfBoundsException when null is present
in the components that must be hashed. Fix this so that hash(['aaa', null])
= hash(['aaa']).

Signed-off-by: Aaron Kimball <aaron@wibidata.com>
Update checkstyle config for latest checkstyle version.

Signed-off-by: Lee Sheng <lsheng@wibidata.com>