Allow a retraction z hop of 0 in timelapse configuration
Allow a retraction z hop of 0 in timelapse configuration

See comment by @markwal at
https://github.com/foosel/OctoPrint/pull/1148#pullrequestreview-14802655

JackGavin13/octoprint-test-not-finished,foosel/OctoPrint,beeverycreative/BEEweb,bicephale/OctoPrint,foosel/OctoPrint,JackGavin13/octoprint-test-not-finished,Salandora/OctoPrint,Salandora/OctoPrint,beeverycreative/BEEweb,beeverycreative/BEEweb,Salandora/OctoPrint,Jaesin/OctoPrint,JackGavin13/octoprint-test-not-finished,Salandora/OctoPrint,foosel/OctoPrint,bicephale/OctoPrint,foosel/OctoPrint,bicephale/OctoPrint,Jaesin/OctoPrint,Jaesin/OctoPrint,JackGavin13/octoprint-test-not-finished,beeverycreative/BEEweb,bicephale/OctoPrint,Jaesin/OctoPrint