Hello,

I've encountered an issue while doing an update to vagrant-libvirt 0.5.3.

```
DEBUG: + echo 'gemspec_file = '\''../vagrant-libvirt-0.5.3.gemspec'\''
DEBUG:
DEBUG:   name = '\''rexml'\''
DEBUG:   requirements = nil
DEBUG:
DEBUG:   type = :runtime
DEBUG:
DEBUG:   spec = Gem::Specification.load(gemspec_file)
DEBUG:   abort("#{gemspec_file} is not accessible.") unless spec
DEBUG:
DEBUG:   dep = spec.dependencies.detect { |d| d.type == type && d.name == name }
DEBUG:   if dep
DEBUG:     dep.requirement.concat requirements
DEBUG:   else
DEBUG:     spec.public_send "add_#{type}_dependency", name, requirements
DEBUG:   end
DEBUG:   File.write gemspec_file, spec.to_ruby'
DEBUG: + ruby
DEBUG: /usr/share/rubygems/rubygems/requirement.rb:146:in `concat': undefined method `flatten' for nil:NilClass (NoMethodError)
DEBUG:  from -:13:in `<main>'
```

This is caused by (I've already removed it; and it works now):

```
-# Rexml needs to be required since Ruby 3.0.
-# https://github.com/vagrant-libvirt/vagrant-libvirt/pull/1277
-%gemspec_add_dep -g rexml -s ../%{vagrant_plugin_name}-%{version}.gemspec

```

As this would be an empty change (rexml was already added), and `requirements` is `nil`, this is not expected use.

Should we enhance the macros to cover this case with a proper error message?

Regards,

Pavel

Additional notes: