Refactor getContextDiffNumbered with separate Nothing equation

[ninioArtillero]

Adding an extra equation for the Nothing case lifts the burden of handling it at doPreffix and doSuffix, resulting in clearer code and avoiding unnecessary traversals of getGroupedDiff a0 b0 output. This is possible because the Nothing case resulted in all elements being preserved, particularly no Both being split, and thus producing a single hunk with the output of getGroupedDiff.

It is also of note that calls to max were already no-ops as drop truncates negatives to 0.