add check for definedness of cookie name

We quite often observe the following warning in our logs
 Use of uninitialized value in hash element at /usr/lib/perl5/site_perl/5.8.8/Dancer/Cookies.pm line 44
which is likely caused by an $env_str containing something like "cookie1=foo; ;cookie2=bar".
Apart from that, $name would be defined but empty if $env_str contains something like " =foo"

Proposed change: check that cookie name in $name is defined and not empty