Issue apache#2362 Evaluate JsonPath functions in the JSON Input transform

[sramazzina]

• JsonPath refuses to evaluate a function path (length(), sum(), keys(), ...)
  while ALWAYS_RETURN_LIST is set and, with SUPPRESS_EXCEPTIONS, returns null
  instead; such a path is now re-evaluated without that option and its scalar
  result exposed as a single row.
• The null returned in that case reached normalizeJsonPathResult and caused a
  NullPointerException while building the error message; the case is now
  handled before it gets there.
• Document JsonPath functions in the JSON Input transform page, including the
  single-row constraint.

Fixes #2362

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Run mvn clean install apache-rat:check to make sure basic checks pass. A more thorough check will be performed on your pull request automatically.
• If you have a group of commits related to the same change, please squash your commits into one and force push your branch using git rebase -i.
• Mention the appropriate issue in your description (for example: addresses #123), if applicable.

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 licensed under the Apache License Version 2.0, January 2004
• In any other case, please file an Apache Individual Contributor License Agreement.

[hansva]

Add some integration tests too please

[sramazzina]

Oh yes I forget I'll do that sorry my bad