[MNG-8060] Use String#isEmpty instead of length

[CrazyHZM]

Following this checklist to help us incorporate your
contribution quickly and easily:

• Make sure there is a JIRA issue filed
  for the change (usually before you start working on it). Trivial changes like typos do not
  require a JIRA issue. Your pull request should address just this issue, without
  pulling in other changes.
• Each commit in the pull request should have a meaningful subject line and body.
• Format the pull request title like [MNG-XXX] SUMMARY,
  where you replace MNG-XXX and SUMMARY with the appropriate JIRA issue.
• Also format the first line of the commit message like [MNG-XXX] SUMMARY.
  Best practice is to use the JIRA issue title in both the pull request title and in the first line of the commit message.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Run mvn clean verify to make sure basic checks pass. A more thorough check will
  be performed on your pull request automatically.
• You have run the Core IT successfully.

If your pull request is about ~20 lines of code you don't need to sign an
Individual Contributor License Agreement if you are unsure
please ask on the developers list.

To make clear that you license your contribution under
the Apache License Version 2.0, January 2004
you have to acknowledge this by using the following check-box.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

• In any other case, please file an Apache Individual Contributor License Agreement.

[cstamas]

@CrazyHZM I think we may conflict, but I'd really like to merge #1413 first as alpha-8 is out.

[CrazyHZM]

@CrazyHZM I think we may conflict, but I'd really like to merge #1413 first as alpha-8 is out.

@cstamas
Don't have any burden, merge yours first, I can resolve the conflict later :)