Datetimes with empty string open interval return 400 instead of 200

[philvarner]

On Planetary Computer, these datetimes return a 400 instead of a 200:

• GET Search with datetime=/1985-04-12T23:20:50.52Z returned status code 400
• GET Search with datetime=1985-04-12T23:20:50.52Z/ returned status code 400

[gadomski]

Needs stac-utils/pgstac#157, at least from the pgstac side.