Return None for unset datetime instead of datetime.min

Per review (@dirkkul): an unset date property should surface as None,
not a datetime.min sentinel. Empty-string input now returns None;
year-0 out-of-range handling is unchanged. Callers propagate the value
as a property (typed Any / Optional[List[Any]]) so None flows through.

Author: devteamaegis

test/test_util.py

@@ -2,6 +2,7 @@
 import uuid as uuid_lib
 from copy import deepcopy
 from datetime import datetime, timedelta, timezone
+from typing import Optional
 from unittest.mock import Mock, patch
 
 import pytest
@@ -458,11 +459,13 @@ def test_is_weaviate_too_old(version: str, too_old: bool):
         ),
         # Test handling year 0 (should return datetime.min)
         ("0000-01-15T14:30:45.123456Z", datetime.min),
-        # Test empty string (protobuf default for unset string field)
-        ("", datetime.min),
+        # Test empty string (protobuf default for unset string field) -> None
+        ("", None),
     ],
 )
-def test_datetime_from_weaviate_str(input_str: str, expected: datetime) -> None:
+def test_datetime_from_weaviate_str(
+    input_str: str, expected: Optional[datetime]
+) -> None:
     assert _datetime_from_weaviate_str(input_str) == expected
 
 

weaviate/util.py

@@ -750,9 +750,9 @@ def _datetime_to_string(value: TIME) -> str:
     return value.isoformat(sep="T", timespec="microseconds")
 
 
-def _datetime_from_weaviate_str(string: str) -> datetime.datetime:
+def _datetime_from_weaviate_str(string: str) -> Optional[datetime.datetime]:
     if not string:
-        return datetime.datetime.min
+        return None
     if string[-1] != "Z":
         string = "".join(string.rsplit(":", 1))