Fix for internal server error on PBC Request import #61950062

Per [story 61950062](https://www.pivotaltracker.com/story/show/61950062).  The problems and their fixes:

Problem: Imports without a status were crashing SQL since this field is not nullable.  
Solution: Make field required in import converter (and do the same for the other two, which weren't set this way).

Problem: Blank Assignee fields were crashing the server instead of defaulting to Audit owner, due to how content read in as `None` is never validated. (Technically, blank common-delimited fields were okay, but if you just cut off the row before it got to a required column, this is interpreted as `None` rather than a blank string.)  
Solution: Ensure that, in the base case, `None` entries are passed to `validate()` in case they are required.  For the particular case of the Assignee field, the validator is updated as follows:

- First, the assignee field is checked for whether it's `None`; if so, it's set to the default.  
- If it's not `None`, the field is outer-whitespace-stripped and then checked for whether the resulting length is zero, and if so, set to the default.  *This avoids the problem of trying to call `strip()` on `None`.