fix: ensure DataFrame has correct columns for empty event streams

When converting an empty event stream, the DataFrame now includes
all expected columns instead of being completely empty. This is
achieved by explicitly defining the columns parameter when creating
the DataFrame.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: goloroden

eventsourcingdb/pandas.py

@@ -20,6 +20,22 @@ async def events_to_dataframe(events: AsyncGenerator[Event, None]) -> pd.DataFra
     Returns:
         A pandas DataFrame with all event fields as columns
     """
+    columns = [
+        "event_id",
+        "time",
+        "source",
+        "subject",
+        "type",
+        "data",
+        "spec_version",
+        "data_content_type",
+        "predecessor_hash",
+        "hash",
+        "trace_parent",
+        "trace_state",
+        "signature",
+    ]
+
     event_list = []
 
     async for event in events:
@@ -40,4 +56,4 @@ async def events_to_dataframe(events: AsyncGenerator[Event, None]) -> pd.DataFra
         }
         event_list.append(event_dict)
 
-    return pd.DataFrame(event_list)
+    return pd.DataFrame(event_list, columns=columns)