Fix: DummyStepperMotor returns None if moving

[alexdewar]

Description

As with the ST10Controller before #743, the DummyStepperMotor returns an angle of None if it is in motion. However, this isn't what we want anymore; if you try to record data while using the DummyStepperMotor you now get an error when the code tries to use this None value. Change it to return the old angle instead.

Type of change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Documentation (non-breaking change that adds or improves the documentation)
• Optimisation (non-breaking, back-end change that speeds up the code)

Key checklist

• Pre-commit hooks run successfully (pre-commit run -a)
• All tests pass (pytest)
• The documentation builds without warnings (mkdocs build -s)
• Check the GUI still works (if relevant)
• Check the code works with actual hardware (if relevant)
• Check the pyinstaller-built executable works (if relevant)

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests have been added or an issue has been opened to tackle that in the future. (Indicate issue here: # (issue))

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.93%. Comparing base (6680116) to head (b2589ee).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #750      +/-   ##
==========================================
- Coverage   86.94%   86.93%   -0.01%     
==========================================
  Files          69       69              
  Lines        3561     3560       -1     
==========================================
- Hits         3096     3095       -1     
  Misses        465      465              

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[dalonsoa]

LGTM!