Should Spacebars.call be modified to always return null

[mitar]

We could add a bit modified version of Spacebars.call which makes sure that all undefined values
are converted to null values. This makes it much easier to work with template inclusions
{{> foobar}}which require inclusion to be a Blaze Template or null, but not undefined.
Additionally, if value is null or undefined, do not complain about not being a function.
This then works much more like CoffeeScript ?, so that {{foo.bar 'test'}} is more like
{{foo?.bar? 'test'}} instead of failing if bar is not defined. In practice, this allows
us to do things like {{> this.lookup.renderEditor}} and {{editor.getValue 'foobar'}}
even if this or editor is undefined.

See meteor/meteor#4230 and meteor/meteor#4232.

This is not a necessary change for Blaze Components to work. But it makes things really simpler sometimes.

Spacebars.call = (value) ->
  if _.isFunction value
    # Evaluate arguments if they are functions (by calling them).
    newArgs = []
    for i in [1...arguments.length]
      arg = arguments[i]
      newArgs.push if _.isFunction(arg) then arg() else arg
    # Always return null instead of undefined.
    value.apply(null, newArgs) ? null

  # If value is not null or undefined, then return it as it is.
  else if value?
    if arguments.length > 1
      throw new Error('Can\'t call non-function: ' + value)
    value

  # But if it is null or undefined, always return null instead.
  else
    null