fix various warnings

fix various warnings

* Use of uninitialized value in pattern match (m//) at C4/Biblio.pm line 1305
  (displaying MARC21 856s that don't have $3)
* Use of uninitialized value in multiplication (*) at catalogue/detail.pl line 243.
  (attemping to calculate average Amazon rating if no Amazon info
   is available)
* opac-detail.pl: Use of uninitialized value in length at C4/External/Amazon.pm line 90
  (attempting to calculate length of $isbn if it happens to be undef)

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>