Python: check if timestamp exists (microsoft#4977)

### Motivation and Context
fix microsoft#4975 

### Description

check if the key timestamp exists before setting it as it's optional

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

python/semantic_kernel/connectors/memory/azure_cosmosdb/mongo_vcore_store_api.py

@@ -101,7 +101,7 @@ async def get(self, collection_name: str, key: str, with_embedding: bool) -> Mem
             text=result["text"],
             description=result["description"],
             additional_metadata=result["metadata"],
-            timestamp=result["timestamp"],
+            timestamp=result.get("timestamp", None),
         )
 
     async def get_batch(self, collection_name: str, keys: List[str], with_embeddings: bool) -> List[MemoryRecord]:
@@ -117,7 +117,7 @@ async def get_batch(self, collection_name: str, keys: List[str], with_embeddings
                 text=result["text"],
                 description=result["description"],
                 additional_metadata=result["metadata"],
-                timestamp=result["timestamp"],
+                timestamp=result.get("timestamp", None),
             )
             for result in results
         ]
@@ -163,7 +163,7 @@ async def get_nearest_matches(
                 text=aggResult["document"]["text"],
                 description=aggResult["document"]["description"],
                 additional_metadata=aggResult["document"]["metadata"],
-                timestamp=aggResult["document"]["timestamp"],
+                timestamp=aggResult["document"].get("timestamp", None),
             )
             if aggResult["similarityScore"] < min_relevance_score:
                 continue