[TW-1558] Field-relative dates resolve incorrectly when no existing date has been assigned to the field

_Jeff Smith on 2015-02-28T01:47:35Z says:_

Consider the command: *task project:Party mod due:due+7d*

The intent of the command is to bump the due date on all Party tasks by 1 week. For tasks that have been assigned a due date, this works as expected. But when the *due* field is empty, the date assigned is 1970-01-08.

Following a discussion in issue #TW-1557, the correct behavior should be for the command to do nothing to the *due* field of undated tasks. That way, bulk lists of tasks can be accelerated or deferred using this date-relative modification syntax without inadvertently assigning due dates to previously undated tasks as a byproduct.