Permit None value for datetime

[alukach]

As mentioned in #77, we should permit a None value for datetime property under certain circumstances.

By the time we are running our validators, we are validating against a Python dictionary (ie not a JSON string). For this reason, we should be permitting None values for properties.datetime when properties.start_datetime and properties.end_datetime are provided.

Going a bit further, I'm not sure that it's correct for us to permit properties.datetime to ever equal "null" (as is currently supported). The STAC Spec states that null is sometimes permitted, however "null" is never mentioned as being permitted.

---

closes #77

Before:

$ git checkout master
Switched to branch 'master'
Your branch is up to date with 'origin/master'.
$ stac-pydantic validate-item https://raw.githubusercontent.com/radiantearth/stac-spec/master/examples/collectionless-item.json

1 validation error for Item
properties -> datetime
  none is not an allowed value (type=type_error.none.not_allowed)

After:

$ git checkout patch-1
Switched to branch 'patch-1'
Your branch is up to date with 'origin/patch-1'.
$ stac-pydantic validate-item https://raw.githubusercontent.com/radiantearth/stac-spec/master/examples/collectionless-item.json

https://raw.githubusercontent.com/radiantearth/stac-spec/master/examples/collectionless-item.json is valid

[alukach]

In 006b0cf, I've dropped support for "null" values. This feels like it may be a it contentious, happy to roll back if desired.

[alukach]

@geospatial-jeff A gentle nudge for this PR, any issues with merging this in?

[ka7eh]

Any estimate on when this will be merged?

[vincentsarago]

will be taken care of in #149