The date range that the date range picker starts with when it opens.
If an initial date range is provided,
initialDateRange.end must both fall between or on firstDate and
lastDate. For all of these DateTime values, only their dates are
considered. Their time fields are ignored.
If initialDateRange is non-null, then it will be used as the initially
selected date range. If it is provided,
initialDateRange.start must be
before or on
final DateTimeRange? initialDateRange;