load_stac: omit time interval in STAC API search request if no temporal extent provided

[bossie]

If a date-time interval is passed to a STAC API search request, VITO STAC APIs will only consider including items for which the datetime property is set; items that lack a datetime and instead specify a start_datetime/end_datetime will not be returned even if they intersect the given interval.

As discussed, the STAC API will probably be adapted to include these items but seeing as it takes time to include these changes in the upstream repo, a quicker fix would be to omit the time interval from the search request; in this case the STAC API will return all items, as one would expect.

One obstacle is the fact that without a given temporal extent, OpenEO will fall back to ["1970-01-01", "2070-01-01"] pretty early in the process graph evaluation.

[bossie]

Related: stac-utils/stac-fastapi-elasticsearch-opensearch#181