Fix filter syntax validation (#8477)

haystack/utils/filters.py

@@ -16,9 +16,9 @@ def raise_on_invalid_filter_syntax(filters: Optional[Dict[str, Any]] = None):
     """
     Raise an error if the filter syntax is invalid.
     """
-    if filters and "operator" not in filters and "conditions" not in filters:
+    if filters and ("operator" not in filters or "conditions" not in filters):
         msg = "Invalid filter syntax. See https://docs.haystack.deepset.ai/docs/metadata-filtering for details."
-        raise ValueError(msg)
+        raise FilterError(msg)
 
 
 def document_matches_filter(filters: Dict[str, Any], document: Document) -> bool: