FIX: Avoid nullrefs from right-clicking when there's no Action Maps (ISXB-833).

[graham-huws]

Description

Fix for ISXB-833, and some extra improvements.

Changes made

Simple nullref avoidance, plus: don't show impossible options (eg. Add Action when there's no Action Maps), and fix the fact that when Actions/Maps were empty, users couldn't right-click the space left by the empty tree/list to add a new item (for Action Maps, this is the "List is empty" text; for Actions the space is unlabelled).

Checklist

Before review:

• [x] Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • FogBugz ticket attached, example ([case %number%](https://issuetracker.unity3d.com/issues/...)).
  • FogBugz is marked as "Resolved" with next release version correctly set.
• [ ] Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• [ ] Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• [x] Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.