facetStripLabel function
The strip label for a facet value under an optional prefix.
A null value renders as facetNullValuePlaceholder (an em dash) rather
than the literal string 'null' — a null value is a valid panel per the
spec, so it gets a defined label. Non-null values stringify as before.
Implementation
String facetStripLabel(Object? value, String? prefix) {
final valuePart = value == null ? facetNullValuePlaceholder : '$value';
return prefix == null ? valuePart : '$prefix: $valuePart';
}