Feature request: allow empty(null) values in RelativeRangeDatePicker presets

[SkiffCMC]

Hello!

Currently, presets props of RelativeRangeDatePicker require both dates(start and end) and sometimes it's convenient to have periods with at least one open date(i.e. "starts from now" or "ends at now+30d").

Suggestion: allow null value in presets which will correspond to empty date in picker.