Use 'n/a' as OS node property if none of the <os> tags have a 'product' value

[rachkor]

Summary

Currently, if the first OS tag for a Node has no product value:

<fingerprints>
    <os certainty="0.80" family="IOS" vendor="Cisco" arch="x86_64"/>
</fingerprints>

Then the upload will crash with an error like:

bundler: failed to load command: thor (/opt/dradispro/dradispro/releases/202306171116/vendor/bundle/ruby/3.1.0/bin/thor)
/opt/dradispro/dradispro/shared/bundle/ruby/3.1.0/bundler/gems/dradis-nexpose-5febb8d166a8/lib/nexpose/node.rb:97:in `method_missing': undefined method `value' for nil:NilClass (NoMethodError)

xml_os.attributes['product'].value
^^^^^^

This PR prevents the error from appearing by selecting the first product value from the <os> tags and if none is found, use n/a

Copyright assignment

I assign all rights, including copyright, to any future Dradis work by myself to Security Roots.