Fix host on gem push, when not passing a --host

After d3f91a011705020c4f1e601d8ddab1c924e5eb99 we start accepting the option --host on the push command
However when the --host is not provided, it is set as nil, and this was causing the following output

Enter your  credentials.
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Merge pull request #622 from arthurnn/fix_push_host

Allow rubygems.org credential creation without --host

Fixes #622