fix: Fix crash when no target_col is specified for Bigframes tensorflow

PiperOrigin-RevId: 602543376

Author: matthew29tang

vertexai/preview/_workflow/serialization_engine/serializers.py

@@ -1284,7 +1284,7 @@ def _deserialize_tensorflow(
         """
         # Set default kwarg values
         batch_size = batch_size or DEFAULT_TENSORFLOW_BATCHSIZE
-        target_col = target_col.encode("ASCII") or b"target"
+        target_col = target_col.encode("ASCII") if target_col else b"target"
 
         # Deserialization at remote environment
         try: