Bug 8434 - Notice generation fails for Advanced Notices, Item Due, and Overdues when...

Bug 8434 - Notice generation fails for Advanced Notices, Item Due, and Overdues when run in shell (due to error in Letters.pm)

Patch introduces a check for C4::Context->userenv in Letters.pm, so that script doesn't fail when it calls to C4::Context->userenv->{branch}, when run from shell.

Without the check, Advanced Notices, Item Due, and Overdues fail to generate.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>