Converting documents fails

I'm getting the following error message when converting e.g. PDF's (on CentOS 7):

```txt
2021-02-01 17:16:20,597 WARNING [waitress.queue:115][MainThread] Task queue depth is 1
2021-02-01 17:16:52,310 INFO [collective.documentviewer:101][waitress] Running command /usr/bin/md5sum /home/plone/some_website/var/blobstorage/0x00/0x00/0x00/0x00/0x00/0x00/0xba/0x5c/0x03dda2a7b2352000.blob
2021-02-01 17:16:52,354 INFO [collective.documentviewer:120][waitress] Finished Running Command /usr/bin/md5sum /home/plone/some_website/var/blobstorage/0x00/0x00/0x00/0x00/0x00/0x00/0xba/0x5c/0x03dda2a7b2352000.blob
2021-02-01 17:16:52,376 ERROR [collective.documentviewer:798][waitress] Error converting PDF:

Traceback (most recent call last):
File "/home/plone/.buildout/eggs/collective.documentviewer-6.0.0-py3.6.egg/collective/documentviewer/convert.py", line 266, in dump_images
qpdf.strip_page(filepath, output_folder)
AttributeError: 'NoneType' object has no attribute 'strip_page'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/plone/.buildout/eggs/collective.documentviewer-6.0.0-py3.6.egg/collective/documentviewer/convert.py", line 770, in __call__
pages = self.run_conversion()
File "/home/plone/.buildout/eggs/collective.documentviewer-6.0.0-py3.6.egg/collective/documentviewer/convert.py", line 591, in run_conversion
return sc.convert(self.storage_dir, **args)
File "/home/plone/.buildout/eggs/collective.documentviewer-6.0.0-py3.6.egg/collective/documentviewer/convert.py", line 493, in convert
gm.dump_images(path, output_dir, sizes, format, language)
File "/home/plone/.buildout/eggs/collective.documentviewer-6.0.0-py3.6.egg/collective/documentviewer/convert.py", line 268, in dump_images
raise Exception
Exception
Traceback (most recent call last):
File "/home/plone/.buildout/eggs/collective.documentviewer-6.0.0-py3.6.egg/collective/documentviewer/convert.py", line 266, in dump_images
qpdf.strip_page(filepath, output_folder)
AttributeError: 'NoneType' object has no attribute 'strip_page'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/plone/.buildout/eggs/collective.documentviewer-6.0.0-py3.6.egg/collective/documentviewer/convert.py", line 770, in __call__
pages = self.run_conversion()
File "/home/plone/.buildout/eggs/collective.documentviewer-6.0.0-py3.6.egg/collective/documentviewer/convert.py", line 591, in run_conversion
return sc.convert(self.storage_dir, **args)
File "/home/plone/.buildout/eggs/collective.documentviewer-6.0.0-py3.6.egg/collective/documentviewer/convert.py", line 493, in convert
gm.dump_images(path, output_dir, sizes, format, language)
File "/home/plone/.buildout/eggs/collective.documentviewer-6.0.0-py3.6.egg/collective/documentviewer/convert.py", line 268, in dump_images
raise Exception
Exception
```