Null valued facets cause the web interface to crash when populating the sidebar

### Describe the bug

It looks like it is failing as there is a facet with the value null, or None in python.

JavaScript can't natively turn a null value into a string, so when it tries it is crashing.

### How to reproduce

Steps to reproduce the behaviour:

1. Visit https://wwwspice/~david.flack/CSET_SCZ_part6/
2. Open the devtools console.
3. Observe the crash.

### Expected behaviour

Instead we need to capture null values, and either filter out facets with null values, or manually convert them to strings.

Reproduce the crash at the linked CSET dataset URL and inspect the browser console while the sidebar facets are populated. Trace the web interface code that handles facet values, then verify that a null-valued facet no longer crashes the sidebar and that the chosen handling matches the expected behavior.

Written by the indexing model from the issue text.