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

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[hansva]

Add some integration tests too please