fix(util): guard _datetime_from_weaviate_str against empty string input

Empty string is the protobuf wire-format default for an unset string
field. Calling string[-1] on "" raises IndexError, crashing gRPC
deserialization for any object with an unset date property. Return
datetime.min for empty input, matching the existing behaviour for
year-zero dates.

Fixes #2052

Author: Ishaan Samantray

test/test_util.py

@@ -458,6 +458,8 @@ 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),
     ],
 )
 def test_datetime_from_weaviate_str(input_str: str, expected: datetime) -> None:

weaviate/util.py

@@ -751,6 +751,8 @@ def _datetime_to_string(value: TIME) -> str:
 
 
 def _datetime_from_weaviate_str(string: str) -> datetime.datetime:
+    if not string:
+        return datetime.datetime.min
     if string[-1] != "Z":
         string = "".join(string.rsplit(":", 1))