Allow empty string values in kibana::config

[smokris]

https://www.elastic.co/guide/en/kibana/current/configuring-tls.html#configuring-tls-kib-es says:

If you used elasticsearch-certutil to generate a PKCS#12 file and you did not specify a password, the file is encrypted, and you need to set server.ssl.truststore.password to an empty string.

This change enables me to use Puppet to set server.ssl.truststore.password to an empty string.

Pull request acceptance prerequisites:

• [x] Signed the CLA (if not already signed)
• [x] Rebased/up-to-date with base branch
• [ ] Updated CHANGELOG.md with patch notes (if necessary) — not a significant change
• [ ] Any relevant docs (README.markdown or inline documentation) updated — not a significant change
• [ ] Updated CONTRIBUTORS
• [x] Tests pass CI