Bug 15869: (QA follow-up) Don't pass placeholder framework '_USE_ORIG'

Bug 15869: (QA follow-up) Don't pass placeholder framework '_USE_ORIG'

This param in the dropdown is meant to signify no change, distinct from
"" when means to set to default. In rebasing this condition was lost, this patch
restores undef of the variable to avoid setting framework to an invalid value

To test:
1 - Stage a record with a match (export and import a record)
2 - Attempt to import and overlay with option 'When replacing records use this framework:' 'Keep original framework'
3 - It fails, logs note 'value too long for field frameworkcode'
4 - Apply this patch
5 - Try again, it succeeds - framework is not changed

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>