*Steps to reproduce:*

{{var asset = ScriptableObject.Create<InputActionAsset>();}}
{{asset.AddActionMap("Map");}}
{{asset.FindAction("Map/NonExistent"); // <--- NullReferenceException here}}

*Actual results:* 

Exception thrown of type NullReferenceException.

*Expected results:* 

null returned from FindAction

*Reproducible with versions:*

Input System 1.8.2 

*Not reproducible with versions:* 

Doesn't seem to be a regression

*Can’t test with versions:* 

N/A

*Tested on (OS):* 

macOS

*Notes:*
* FindAction should have same semantics regardless of used search string.
* Either null checking is needed in this function or InputActionMap without actions should be created with an array of size 0.