FIX: ISXB-895 Fixed an issue where `InputActionAsset.FindAction` would unexpectedly throw `System.NullReferenceException`.

[ekcoh]

Description

Fixed an issue where InputActionAsset.FindAction(string, bool) would throw System.NullReferenceException instead of returning null if searching for a non-existent action with an explicit action path and using throwIfNotFound: false, e.g. searching for "Map/Action" when InputActionMap "Map" exists but no InputAction named "Action" exists within that map ISXB-895.

Changes made

Added a null check to actions array for path-evaluation in FindActions.

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.
• [x] Tests added/changed, if applicable.
  • Functional tests: Actions_CanAttemptToLookUpNonExistentActionInAssetByPath.
  • Performance tests: Not expecting any noticeable differences.
  • Integration tests: N/A
• [ ] 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:

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