Handle join_keys_to_values() with undef values.

[BobVanB]

support flags

gitlab_ci_runners_defaults:
  tag-list: 'custom'
  run-untagged:
  executor: 'shell'

(gitlab module joins on space)
would become:
--tag-list custom --run-untagged --executor shell

the --run-untagged does not accept values.

Because puppet and hieradata returns string values i had to use String(v) == "undef".
I couldn't get it to work with not defined?(v) or v.nil?.