gitweb: Protect escaping functions against calling on undef

This is a bit of future-proofing esc_html and friends: when called
with undefined value they would now would return undef... which would
probably mean that error would still occur, but closer to the source
of problem.

This means that we can safely use
  esc_html(shift) || "Internal Server Error"
in die_error() instead of
  esc_html(shift || "Internal Server Error")

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>