fix: prevent insert error when creating deferred transaction

[Chadys]

create_deferred_transaction is using None as default for reference and status. Those are both fields that can be blank but not null in DB.
I replaced the defaults of the function to '' so that the object can actually be saved to the DB when not specifying them.

[codecov]

Codecov Report

Merging #4288 (51fdfb4) into master (204b9a7) will increase coverage by 0.07%.
Report is 11 commits behind head on master.
The diff coverage is n/a.

❗ Current head 51fdfb4 differs from pull request most recent head d2b65ab. Consider uploading reports for the commit d2b65ab to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4288      +/-   ##
==========================================
+ Coverage   88.28%   88.36%   +0.07%     
==========================================
  Files         293      293              
  Lines       16046    16094      +48     
==========================================
+ Hits        14166    14221      +55     
+ Misses       1880     1873       -7     

Files	Coverage Δ
src/oscar/apps/payment/abstract_models.py	89.55% <ø> (ø)

... and 9 files with indirect coverage changes