Avoid exception during print start

[alexiri]

At the beginning of the print, I'm seeing the following errors:

2024-07-14 17:22:12,139 - octoprint.plugins.detailedprogress - INFO - Message: ETA: No ETA yet
2024-07-14 17:22:12,141 - octoprint.plugins.detailedprogress - INFO - Caught an exception unsupported operand type(s) for /: 'NoneType' and 'int'
Traceback:Traceback (most recent call last):
  File "/home/pi/oprint/lib/python3.9/site-packages/octoprint_detailedprogress/__init__.py", line 80, in do_work
    self._update_progress(currentData)
  File "/home/pi/oprint/lib/python3.9/site-packages/octoprint_detailedprogress/__init__.py", line 88, in _update_progress
    printMinutesLeft = int(currentData["progress"]["printTimeLeft"] / 60)
TypeError: unsupported operand type(s) for /: 'NoneType' and 'int'

2024-07-14 17:22:22,144 - octoprint.plugins.detailedprogress - INFO - Message: 0.31% complete
2024-07-14 17:22:22,147 - octoprint.plugins.detailedprogress - INFO - Caught an exception unsupported operand type(s) for /: 'NoneType' and 'int'
Traceback:Traceback (most recent call last):
  File "/home/pi/oprint/lib/python3.9/site-packages/octoprint_detailedprogress/__init__.py", line 80, in do_work
    self._update_progress(currentData)
  File "/home/pi/oprint/lib/python3.9/site-packages/octoprint_detailedprogress/__init__.py", line 88, in _update_progress
    printMinutesLeft = int(currentData["progress"]["printTimeLeft"] / 60)
TypeError: unsupported operand type(s) for /: 'NoneType' and 'int'

This commit should avoid them.