Fixed NPE (rebased onto develop)



This is the same as gh-2589 but rebased onto develop.

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Just fixed a NPE, see https://trac.openmicroscopy.org.uk/ome/ticket/12365
Don't know how to test (how to get into the state that originalDef is not set), maybe a code review is sufficient?
Just applied the same logic to isOriginalPlane() as isOriginalSettings(), i. e. originalDef==null implies nothing has changed, return true.