Remove empty values from CVE filter params

[mtruj013]

Done

• Removed default empty values for version and priority param

QA

• View the site locally in your web browser at: https://ubuntu-com-14027.demos.haus/security/cves
• Click search and see that empty version and priority values are not passed by default and that the CVE list remains unchanged
• Using the form, search for the package "mysql" and see that you get 1359 results
• Do the same thing but for package "ssh" and see that you get 139 results

Issue / Card

Fixes https://github.com/canonical/ubuntu.com/issues/14020, https://warthogs.atlassian.net/browse/WD-12819

[webteam-app]

Demo

Jenkins

demos.haus

[lizzochek]

Please fix lint Jinja before merging