search_by_cve and filter hits bug

[cerrussell]

The search_by_cve function uses "*" as the version when it calls vers_compare. However, vers_compare does not check for this condition and will therefore return no results when what is desired is all results.

ETA - This was resolved by @prabhu , however small change is desirable to check for NoneType rather than that the value is None (as that will catch both None and empty string).