Check for legend existing before using in _correct_for_scroll_event_on_legend

[jhaigh0]

Description of work

Found when doing engineering diffraction manual tests
#37992 (comment)

When scrolling on the plots created by the engineering diffraction interface (which have no legends), the following error occurred.

Traceback (most recent call last):
File "C:\MantidNightlyInstall\bin\lib\site-packages\matplotlib\cbook_init_.py", line 314, in process
func(*args, **kwargs)
File "C:\MantidNightlyInstall\bin\lib\site-packages\workbench\plotting\figureinteraction.py", line 141, in on_scroll
self._correct_for_scroll_event_on_legend(event)
File "C:\MantidNightlyInstall\bin\lib\site-packages\workbench\plotting\figureinteraction.py", line 174, in _correct_for_scroll_event_on_legend
if legend.get_draggable() and legend.contains(event):
AttributeError: 'NoneType' object has no attribute 'get_draggable'

There is no associated issue.

To test:

• Turn on ISIS archive
• Open Diffraction -> Engineering Diffraction interface
• Select Create New Calibration and enter 305738
• Click Calibrate and scroll on the plot which appears, no error should occur.

This does not require release notes because changes added this release

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[yusufjimoh]

Thank you for the quick fix. I was able to scroll plots with and without legends, including those with hidden legends, and the scroll works perfectly with no side effects. I'm happy to approve the pull request