RFR: 2182: Throw UncheckedRestException on empty GET responses [v2]

[Erik Joelsson]

> A very common type of SEVERE logging message, which we get notifications for as admins, is this:
> 
> Exception during periodic items checking: Cannot invoke "org.openjdk.skara.json.JSONArray.stream()" because the return value of "org.openjdk.skara.json.JSONValue.asArray()" is null
> 
> There are some other kinds of instances where the NPE isn't providing any information, but that are likely caused by the same thing. From what I can tell, what's happening is that a REST GET call (typically to GitHub) returns an empty string in the response body. Our JSON parser translates this to a JSONNull and we subsequently try to treat it as a JSONArray and get the NPE when trying to call `stream()` on it. This typically happens a few times per day across all the bots. Ultimately I think this is a bug with GitHub, but we need to deal with it in a better way.
> 
> My proposed solution is to add an option to RestRequest, `failOnEmptyResponse`, that defaults to true for GET requests but otherwise false. If set and the response body is empty, an UncheckedRestException is thrown. This exception is already handled as just a warning and we are collecting metrics for it to sound alarms if we get a lot of them.

Erik Joelsson has updated the pull request incrementally with three additional commits since the last revision:

 - Only include the redirect in the RestException message if Location header is present
 - Include Location in RestException message
 - Improve logging of RestException

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Changes:
  - all: https://git.openjdk.org/skara/pull/1614/files
  - new: https://git.openjdk.org/skara/pull/1614/files/a1527449..e72f3612

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1614&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1614&range=00-01

  Stats: 6 lines in 2 files changed: 3 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/skara/pull/1614.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1614/head:pull/1614

PR: https://git.openjdk.org/skara/pull/1614