fix(reprot): use SQLite3 in current dir if not specified

[kotakanbe]

What did you implement:

Use SQLite3 in current dir if not specified in config.toml

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Report without specifying the path of SQLite3 in config.toml.
Confirmed that SQLite3 in the current directory is used.

 ✗  ubuntu@dev  ~│g│s│g│f│vuls  ⎇ master~  ./vuls tui --refresh-cve
[Dec 26 07:54:24] ERROR Failed to create log file. path: /var/log/vuls/localhost.log, err: open /var/log/vuls/localhost.log: permission denied
[Dec 26 07:54:24]  INFO [localhost] Validating config...
INFO[0000] -cvedb-type: sqlite3, -cvedb-url: , -cvedb-path: /home/ubuntu/go/src/github.com/future-architect/vuls/cve.sqlite3
[Dec 26 07:54:24]  INFO [localhost] Loaded: /home/ubuntu/go/src/github.com/future-architect/vuls/results/2020-12-26T07:22:12+09:00
[Dec 26 07:54:24]  INFO [localhost] Validating db config...
INFO[0000] -cvedb-type: sqlite3, -cvedb-url: , -cvedb-path: /home/ubuntu/go/src/github.com/future-architect/vuls/cve.sqlite3
INFO[0000] -ovaldb-type: sqlite3, -ovaldb-url: , -ovaldb-path: /home/ubuntu/go/src/github.com/future-architect/vuls/oval.sqlite3
INFO[0000] -gostdb-type: sqlite3, -gostdb-url: , -gostdb-path: /home/ubuntu/go/src/github.com/future-architect/vuls/gost.sqlite3
INFO[0000] -exploitdb-type: sqlite3, -exploitdb-url: , -exploitdb-path: /home/ubuntu/go/src/github.com/future-architect/vuls/go-exploitdb.sqlite3
INFO[0000] -msfdb-type: sqlite3, -msfdb-url: , -msfdb-path: /home/ubuntu/go/src/github.com/future-architect/vuls/go-msfdb.sqlite3

Checklist:

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES