Guard against content being None

`content` can be passed in as None, so we need to set it to the empty
list if that occurs.

This also fixes the bad-practice of using a mutable data-type as a
default argument.

Fixes bug 1163598

Change-Id: Ie991c9452a21cfb036d3955a094ec345805d63d1
Merge "Guard against content being None"