The _merge_head_data() function only called calculate_total_score() for
links present in url_to_head_data. Links that failed head extraction
(PDFs, timeouts, non-HTML) hit the else branch and were appended
unchanged, leaving total_score as None even when intrinsic_score was
available.
Added calculate_total_score() calls in both else branches (internal
and external links) so all links get a total_score computed from their
intrinsic_score when head data is unavailable.
Fixes#1749
