Fix TypeError with Click Sentinel in Python 3.14 (#723)

Co-authored-by: Dror Shvadron <dror.shvadron@rotman.utoronto.ca>

Author: DrorSh

src/_pytask/mark/__init__.py

@@ -92,12 +92,14 @@ def pytask_extend_command_line_interface(cli: click.Group) -> None:
             metavar="MARKER_EXPRESSION",
             type=str,
             help="Select tasks via marker expressions.",
+            default=None,
         ),
         click.Option(
             ["-k", "expression"],
             metavar="EXPRESSION",
             type=str,
             help="Select tasks via expressions on task ids.",
+            default=None,
         ),
     ]
     for command in ("build", "clean", "collect"):