Set cvss vector default to nil

What does this MR do and why?

The cvss vector validator treats attributes with empty string as invalid. This change updates the data object created to set cvss vectors to nil if they are missing from the imported json.

Changelog: changed

How to set up and validate locally

Run below in rails console.

json = '{"advisory":{"id":"a2e11cbc-61b8-4b91-b72b-fe5df1bc492b","source":"glad","cvss_v2":"AV:N/AC:M/Au:N/C:P/I:P/A:P","published_date":"2013-11-02","identifiers":[{"type":"cve","name":"CVE-2013-4457","value":"CVE-2013-4457","url":"https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2013-4457"}]},"packages":[{"name":"cocaine","purl_type":"gem","affected_range":">=0.4.0 <=0.5.2","solution":"Upgrade to 0.5.3","fixed_versions":["0.5.3"]}]}'
obj = PackageMetadata::AdvisoryDataObject.create(JSON.parse(json), 'rubygem')
advisory = PackageMetadata::Advisory.new(advisory_xid: obj.advisory_xid, cvss_v2: obj.cvss_v2, cvss_v3: obj.cvss_v3, source_xid: 'glad', published_date: obj.published_date, identifiers: obj.identifiers)
advisory.valid?

For master, the json input incorrectly generates an invalid advisory because cvss_v3 is missing in the json. For this branch, a valid advisory is generated.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #417914 (closed)