Bug 12916 - Use of uninitialized values $max and $min in C4/Tags.pm

Bug 12916 - Use of uninitialized values $max and $min in C4/Tags.pm

When the stratify_tags function is called without any tags, the
$min and $max variables are left undefined, which triggers a
warning message about uninitialized values in the subtraction.

TEST PLAN
---------
1) prove -v t/db_dependent/Tags.t
   -- should succeed, but there is no test for empty tags.
2) Make sure that you have no tags
3) In Opac, go to 'Tag cloud'
4) Check the opac error log file
   (e.g. ~/koha-dev/var/log/koha-opac-error_log) for entries
   regarding uninitialized values $max and $min in C4/Tags.pm
5) apply this patch
6) prove -v t/db_dependent/Tags.t
   -- should still succeed, though no tests are added.
7) Reload page 'Tag cloud'
   -- Result: No additional warnings in opac error log file.

Signed-off-by: Chris <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
New and old tests pass. Works as described.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>