Wrench fails to write required key datetime if it cannot be determined from the file.

[kylemann16]

https://github.com/PDAL/wrench/blob/main/src/vpc.cpp#L371-L372

In the creation of a VPC, which is at its core a STAC ItemCollection, Wrench fails to write the required key datetime if the date cannot be parsed

I think either an exception should be thrown, or a default datetime should be written to the VPC.

STAC Item Schema for reference: https://schemas.stacspec.org/v1.0.0/item-spec/json-schema/item.json