[bugfix] Treat correctly failed_stage=None in failure stats report

[xinye-hpe]

This PR tries to address the following error when using the --failure-stats command line option

ERROR: run session stopped: type error: unsupported format string passed to NoneType.__format__
ERROR: Traceback (most recent call last):
  File "/hpcdc/project/scilib/libsci_acc/csml-libsci-acc-testing/python-libs/x86_64/python3.11/site-packages/reframe/frontend/cli.py", line 1654, in main
    printer.failure_stats(
  File "/hpcdc/project/scilib/libsci_acc/csml-libsci-acc-testing/python-libs/x86_64/python3.11/site-packages/reframe/frontend/printer.py", line 227, in failure_stats
    stats_body.append(row_format.format(p, len(l), l[0]))
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: unsupported format string passed to NoneType.__format__

The problem is that successful tests are stored with tc['fail_phase'] as None and also got passed to the dictionary failures. Then later when ReFrame tries to generate a report, p is set to None in the following lines

for p, l in failures.items():
    stats_body.append(row_format.format(p, len(l), l[0]))

You can reproduce the same behavior with

$ python -c '"{:<13}".format(None)'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: unsupported format string passed to NoneType.__format__

[vkarak]

Thanks! I have just a minor style comment.