InputActionAsset.FindAction throws exception if map is empty. (EditMode)

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.