Fix de_dot to process keys with falsey values
Check for key existence rather than key truthiness.
Functionally a backport of the (as yet) unmerged
https://github.com/logstash-plugins/logstash-filter-de_dot/pull/11
upstream by Tom Brown <tombrown52@gmail.com>.
Bug: T136001
Change-Id: I86b49a2c8232a509b956db158e5e07adcb3840d4