Permit None value for datetime

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

Author: alukach

stac_pydantic/item.py

@@ -16,11 +16,11 @@ class ItemProperties(StacCommonMetadata):
     https://github.com/radiantearth/stac-spec/blob/v1.0.0/item-spec/item-spec.md#properties-object
     """
 
-    datetime: Union[dt, str] = Field(..., alias="datetime")
+    datetime: Union[dt, str, None] = Field(..., alias="datetime")
 
     @validator("datetime")
-    def validate_datetime(cls, v: Union[dt, str], values: Dict[str, Any]) -> dt:
-        if v == "null":
+    def validate_datetime(cls, v: Union[dt, str, None], values: Dict[str, Any]) -> dt:
+        if v in ("null", None):
             if not values["start_datetime"] and not values["end_datetime"]:
                 raise ValueError(
                     "start_datetime and end_datetime must be specified when datetime is null"