Bug 7862: Preventing warns when creating a notice

Bug 7862: Preventing warns when creating a notice

To reproduce warns:
1) Go to Tools -> Notices & slips
2) Create a new notice of any module

Notice these warns:
Use of uninitialized value $code in regexp compilation at
/home/vagrant/kohaclone/tools/letter.pl line 265.
Use of uninitialized value $code in regexp compilation at
/home/vagrant/kohaclone/tools/letter.pl line 265.
Use of uninitialized value $code in regexp compilation at
/home/vagrant/kohaclone/tools/letter.pl line 265.

CGI::param called in list context from package
CGI::Compile::ROOT::home_vagrant_kohaclone_svc_letters_get line 50, this
can lead to vulnerabilities. See the warning in "Fetching the value or
values of a single named parameter" at /usr/share/perl5/CGI.pm line 436.
CGI::param called in list context from package
CGI::Compile::ROOT::home_vagrant_kohaclone_svc_letters_get line 50, this
can lead to vulnerabilities. See the warning in "Fetching the value or
values of a single named parameter" at /usr/share/perl5/CGI.pm line 436.

3) Create a new notice of the Circulation module

Notice this additional warn:
Use of uninitialized value $code in pattern match (m//) at
/home/vagrant/kohaclone/tools/letter.pl line 258.

To test:
4) Apply patch and refresh page
5) Run steps 1-3 again and confirm no warns show

Sponsored-by: Catalyst IT
Signed-off-by: Maryse Simard <maryse.simard@inlibro.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>