allow null, undefined, if and unless for strict component arguments

[mydea]

I am not 100% sure, but I think that this should be correct and fixes #921.

With this PR, the following would be allowed with strictComponents=true:

<PowerSelect
  @afterOptionsComponent={{if 
    @afterOptionsComponent 
    (ensure-safe-component @afterOptionsComponent) 
    null
  }}
  @afterOptionsComponent={{null}}
  @afterOptionsComponent={{undefined}}
  @afterOptionsComponent={{unless @thing (ensure-safe-component @afterOptionsComponent)}}
  @afterOptionsComponent={{ensure-safe-component @afterOptionsComponent}}
/>

Basically, it specifically allows null, undefined, ensure-safe-component, if and unless. When using if or unless, it will check if each path either returns null/undefined or a valid component. So the following will still fail:

<PowerSelect
  @afterOptionsComponent={{if @check 'string'}}
  @afterOptionsComponent={{if @check (component @afterOptionsComponent)}}
/>

I added a bunch of tests hopefully covering this well.

[ef4]

Thanks, this seems good but haven't had a chance to fully review these changes yet.

I do want to point out that this usage:

  @afterOptionsComponent={{if 
    @afterOptionsComponent 
    (ensure-safe-component @afterOptionsComponent) 
    undefined
  }}

shouldn't be necessary because ensure-safe-component tolerates null and undefined by passing them through.

But in general, this is a nice extension to the rules system.

[mydea]

I do want to point out that this usage:

  @afterOptionsComponent={{if 
    @afterOptionsComponent 
    (ensure-safe-component @afterOptionsComponent) 
    undefined
  }}

shouldn't be necessary because ensure-safe-component tolerates null and undefined by passing them through.

Ah, true, the example is actually "wrong", I wanted to show it with null as fallback, which is used in power select (where it differentiates between undefined = default component, null = no component).