Project recovery prevents mantid 6.9.0 opening if it previously crashed in <6.9.0

[thomashampson]

Describe the bug
Project recovery prevents mantid 6.9.0 opening if it previously crashed in <6.9.0

FrameworkManager-[Notice] Welcome to Mantid 6.9.0rc1
FrameworkManager-[Notice] Please cite: http://dx.doi.org/10.1016/j.nima.2014.07.029 and this release: http://dx.doi.org/10.5286/Software/Mantid
DownloadInstrument-[Notice] All instrument definitions up to date
Traceback (most recent call last):
  File "/home/tom/mambaforge/envs/mantid6.9_env/lib/python3.10/site-packages/workbench/app/start.py", line 206, in create_and_launch_workbench
    main_window.project_recovery.attempt_recovery()
  File "/home/tom/mambaforge/envs/mantid6.9_env/lib/python3.10/site-packages/workbench/projectrecovery/projectrecovery.py", line 276, in attempt_recovery
    self.loader.attempt_recovery()
  File "/home/tom/mambaforge/envs/mantid6.9_env/lib/python3.10/site-packages/workbench/projectrecovery/projectrecoveryloader.py", line 36, in attempt_recovery
    self.recovery_presenter = ProjectRecoveryPresenter(self.pr)
  File "/home/tom/mambaforge/envs/mantid6.9_env/lib/python3.10/site-packages/workbench/projectrecovery/recoverygui/projectrecoverypresenter.py", line 17, in __init__
    self.model = model if model else ProjectRecoveryModel(project_recovery, self)
  File "/home/tom/mambaforge/envs/mantid6.9_env/lib/python3.10/site-packages/workbench/projectrecovery/recoverygui/projectrecoverymodel.py", line 31, in __init__
    self.fill_rows()
  File "/home/tom/mambaforge/envs/mantid6.9_env/lib/python3.10/site-packages/workbench/projectrecovery/recoverygui/projectrecoverymodel.py", line 170, in fill_rows
    self._fill_row(path, checkpoint_name)
  File "/home/tom/mambaforge/envs/mantid6.9_env/lib/python3.10/site-packages/workbench/projectrecovery/recoverygui/projectrecoverymodel.py", line 199, in _fill_row
    num_of_ws = str(self.find_number_of_workspaces_in_directory(path))
  File "/home/tom/mambaforge/envs/mantid6.9_env/lib/python3.10/site-packages/workbench/projectrecovery/recoverygui/projectrecoverymodel.py", line 48, in find_number_of_workspaces_in_directory
    rec_file_dict = json.load(reader)
  File "/home/tom/mambaforge/envs/mantid6.9_env/lib/python3.10/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/home/tom/mambaforge/envs/mantid6.9_env/lib/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/home/tom/mambaforge/envs/mantid6.9_env/lib/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/home/tom/mambaforge/envs/mantid6.9_env/lib/python3.10/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

To Reproduce

1. Open mantidworkbench 6.8
2. set log level to debug
3. open a workspace
4. wait for project recovery to run
5. run segfault to cause a crash
6. attempt to open mantidworkbench 6.9

Expected behavior
workbench should launch and warn the user that project recovery isn't possible (ideally it would be able to recover the project, but that might be an unfeasible amount of work and add a lot of code).

Platform/Version (please complete the following information):

• OS: all (I think)
• Mantid Version 6.9.0rc1

Additional context
Introduced by #36859

[github-actions]

Closed by #36914.