Problem: Default thumbnails are not generated in normalization

Expected behaviour

All modes of thumbnail normalization work.

Current behaviour

According to the Generate thumbnails section of the documentation when Yes is selected as the value of the Choose thumbnail mode processing configuration setting:

thumbnails will be generated according to the format rules in the FPR. Formats which do not have a rule will have a default thumbnail generated (grey document icon).

However, in the Ingest tab the Normalize -> Normalize for thumbnails job is marked as successful but the tasks for those formats without rules show an error like this:

  File "/tmp/547569e3-17e1-474b-a6ee-00bb1c55616c", line 8
    DEFAULT_THUMBNAIL = \"\"\"
                             ^
SyntaxError: unexpected character after line continuation character
Failed: Using default thumbnail
Standard out: 
Standard error:   File "/tmp/547569e3-17e1-474b-a6ee-00bb1c55616c", line 8
    DEFAULT_THUMBNAIL = \"\"\"
                             ^
SyntaxError: unexpected character after line continuation character

[Errno 2] No such file or directory: '/var/archivematica/sharedDirectory/currentlyProcessing/demo-03f22bcc-47a6-4e95-9dce-d4ba59902d8d/thumbnails/5745c15a-8ae4-4bdc-86ca-492d1331f426.jpg'

Steps to reproduce

Create a transfer using the SampleTransfer/DemoTransferCSV sample transfer.
Set the Choose thumbnail mode processing configuration setting as Yes.
See the output of the Normalize -> Normalize for thumbnails job in the Ingest tab and notice the failed tasks for the piiTestDataCreditCardNumbers.txt and bird.mp3 files.
Check the thumbnails folder of the AIP and notice the piiTestDataCreditCardNumbers.txt and bird.mp3 files don't have thumbnail files.

Your environment (version of Archivematica, operating system, other relevant details)

https://github.com/artefactual/archivematica/commit/37ba46a6f90d64282f9f8f94acc635949c4c52a1 but also reproduced in the Archivematica sandbox which is currently running 1.13.2.

For Artefactual use:

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[ ] All pull requests related to this issue are properly linked
[ ] All pull requests related to this issue have been merged
[ ] A testing plan for this issue has been implemented and passed (testing plan information should be included in the issue body or comments)
[ ] Documentation regarding this issue has been written and merged (if applicable)
[ ] Details about this issue have been added to the release notes (if applicable)

archivematica / Issues

Problem: Default thumbnails are not generated in normalization #1682