Fix bug where -t argument was processed as filename

Fix bug where -t argument was processed as filename

Fixed issue where `scryer-prolog -t halt` would try to load "halt.pl"
as a file instead of just using halt as the custom toplevel.

The bug was caused by an extra clause `delegate_task([], []).` that
would return control to the calling context instead of continuing to
start_toplevel. This caused the argument processing in delegate_task
to continue and treat the already-consumed toplevel argument as a
filename.

Removing this clause ensures that delegate_task([], Goals0) always
proceeds to load initialization files and start the toplevel, fixing
the double-processing bug.

Co-Authored-By: J.J.'s Robot <[email protected]>