feat #168 - Allow empty pathways in visualize_pathways_levelX func

[SaarJunius]

For reference: inbo/alien-species-portal#186

Changes to visualize_pathways_levelX functions:

1. Allow for the pathway parameter to have elements that are not in the data
2. Make sure each element in the pathway parameter has a (possibly empty) bar

[SaarJunius]

Quite a few tests in the build are failing and producing errors similar to this one (and other class related errors).
plot_output_absolute$static_plot has type 'object', not 'list'.
However, this failure already seems to be present in the main branch and does not appear to be related to any changes in this PR.
How should we proceed?