Allow null values to be returned from Macros

This change allows a Macro returns a 'null' value (as opposed to None)
that makes it effectively cease to exist when that makes sense. If it
appears as a list item, that list item is dropped. If it appears as a
value in a dict, the corresponding key is removed from the dict. If it
appears as a property value, the property is treated as if the user had
not mentioned it in the template.

In all other circumstances (as an argument to a Function, as the top
level of some other field, like an output value), the result will still
appear as None.

A null value is represented internally by the Ellipsis singleton (to
distinguish it from None, which is a value that may appear in the
template from a user or be returned from a Function).

Change-Id: Iaed0982e0db902f6eaf8f986c12b4885bd77e8b9
Story: 2007388
Task: 38976

0 files changed, 0 insertions, 0 deletions