fix interpolation of None value

interpolation use string.Template that always return str.
In most case, the caller convert the value to the option type.
But that doesn't work if the value is None.

This change make it works for None by replacing it by an empty string.

Closes-bug: #1654621
Change-Id: I9e318050d26277df99c9352dca70dc4ef4fbe13b