Prevent crash if CPAN distribution has blank author field

[NicholasBHubbard]

This PR fixes issue #1523.

Because Class::Data::Inheritable version 0.08 has a blank author field in its META.yml file, the bug can be reproduced with the following command:

fpm --no-cpan-test --cpan-verbose --verbose --debug-workspace --workdir $HOME/fpm_tmp/ -s cpan -t rpm -v 0.08 Class::Data::Inheritable

[wbraswell]

As the original bug identifier and fix author, I approve of this PR.
This PR replaces one part of my original now-closed PR:
#1528
@jordansissel: Please accept this PR, thanks! :-)

[jordansissel]

Looks good!

I tested as recommended in the description:

% bundle exec bin/fpm --no-cpan-test --cpan-verbose --verbose --debug-workspace -s cpan -t rpm -v 0.08 Class::Data::Inheritable
...
% rpm -qp perl-Class-Data-Inheritable-0.08-1.noarch.rpm --qf "%{Vendor}\n"
No Vendor Or Author Provided

I confirmed on the main branch that the same fpm command line fails:

Invalid package configuration: Unexpected CPAN 'author' field type: NilClass. This is a bug. {:level=>:error}

[jordansissel]

fpm 1.15.0 is released and contains this improvement.