Search datetime parameter does not support empty string for start or end of an interval

[philvarner]

Per http://docs.opengeospatial.org/is/17-069r3/17-069r3.html#_parameter_datetime

Open ranges in time intervals at the start or end are supported using a double-dot (..) or an empty string for the start/end.

So, both ../1985-04-12T23:20:50.52Z and /1985-04-12T23:20:50.52Z should be supported as an interval, and likewise 1985-04-12T23:20:50.52Z/