Bug 16179: Do not crash if "rate me" is clicked and not rate selected

Bug 16179: Do not crash if "rate me" is clicked and not rate selected

If JS is disabled and a user clicks on the "Rate me" button, Koha will
crashes with:
DBIx::Class::ResultSet::create(): Column 'rating_value' cannot be null
at /usr/share/koha/lib/C4/Ratings.pm line 208

To avoid that, opac-ratings.pl will check if a rate has been selected.

Test plan:
Disable JS
On a record detail page, click on the "Rate me" button

TESTED PLAN:
1) go to /cgi-bin/koha/opac-ratings.pl?biblionumber=1
   -- kaboom as above.
2) apply patch
3) refresh
   -- either login screen (don't know why)
   -- or if already logged in, detail page.
4) koha qa test tool

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
I tested successfully by temporarily removing the modification made by
Bug 16210.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher <bredan@bywatersolutions.com>