There are cases where a series or a series category is identified by null value.
This can happen when rendering a categorical bar chart were on x value is null or when creating a chart with a splitSeriesAccessor with a null value (see #594).
How we should treat them?

1. remove this value or the series completely from the rendering
2. treat them as a special case, with an assigned Unknown, n/a label or similar?

Do we need to handle that in the library or shall the consumer handle this case?