Fix appscan import failure on empty proof

[adfoster-r7]

Closes #14209

Verification

Verify the appscan import now works; Previously the following error would be raised:

[-] Error while running command db_import: Validation failed: Proof can't be blank

Call stack:
/home/gwillcox/.rvm/gems/ruby-2.6.6@metasploit-framework/gems/activerecord-5.2.4.4/lib/active_record/validations.rb:80:in `raise_validation_error'

Now there's no error

[cgranleese-r7]

Release Notes

This PR fixes an issue were an appscan import would fail due to an empty proof.