Consider empty array documented when a field beneath it is documented

[sharanggupta]

Fixes gh-626

When a field beneath an array is documented, for example items[].k2[].name, removing it from the payload cascades upwards and removes any container that the removal leaves empty. An items[] entry whose k2 array is empty to begin with is never reached by this cascade because it has no name entry to remove, so the empty array is left behind and the payload is reported as having undocumented content ({"items":[{"k2":[]}]}) even though every field in it has been documented.

This change makes JsonFieldProcessor notify its callback when traversal reaches an empty collection through a non-leaf array segment. The removal callbacks remove the empty collection as they would a matched leaf, so it and any containers it leaves empty are removed. The callbacks that determine a field's presence (hasField) and extract values for type discovery (extract) ignore the notification, so their behaviour is unchanged.

Scope is deliberately limited to empty arrays, as described in the issue. The analogous mixed-presence cases of a null or an empty object on the path are left as they are; happy to look at those too if you think they belong in the same change.

Tests cover remove and removeSubsection on sometimes-empty and always-empty arrays, the exact payload from the issue via JsonContentHandler#getUndocumentedContent, and an end-to-end ResponseFieldsSnippet rendering. The spring-restdocs-core suite (907 tests), checkFormat and checkstyle pass.