Check whether the value of a dateRangeInput is valid
Source:R/utils_validation.R
      valid_dateRangeInput.RdBoth dates need to be supplied for the value to be considered valid, and the start date needs to be on or before the end date
R/utils_validation.R
      valid_dateRangeInput.RdBoth dates need to be supplied for the value to be considered valid, and the start date needs to be on or before the end date