Merge topic 'command-arg-parser-maybe-empty'

f46b2e9142 cmArgumentParser: Model maybe-missing string with wrapper type
e6d1e29ffa cmArgumentParser: Model maybe-empty and non-empty lists with wrapper types

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Merge-request: !7459