Gerrit Trigger configuration should honor central gerrit configuration.

Formerly, build status values that where not set where hardcoded in
GerritContext, so even if you defined different default values in your
global gerrit trigger configuration (like CR +2 for sucessful builds),
they would be overriden.

This commit sets default value null  and only creates
config entries, when value is not null

see https://issues.jenkins-ci.org/browse/JENKINS-23318