Update src/session_store.py

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: badMade

src/session_store.py

@@ -17,13 +17,13 @@ class StoredSession:
 
 
 def validate_session_id(session_id: str) -> None:
+    if not session_id:
+        raise ValueError("Invalid session ID: cannot be empty")
     if "/" in session_id or "\\" in session_id:
         raise ValueError(f"Invalid session ID: contains path separators ({session_id})")
-    if session_id in (".", ".."):
-        raise ValueError(f"Invalid session ID: cannot be '.' or '..' ({session_id})")
-    if ".." in session_id:
+    if session_id == "." or ".." in session_id:
         raise ValueError(
-            f"Invalid session ID: contains directory traversal ('..') ({session_id})"
+            f"Invalid session ID: contains directory traversal markers ({session_id})"
         )