fix(grouping): Fix FallbackVariant.description property

[lobsterkatie]

When we calculate event grouping, we generate instances of various BaseVariant subclasses. Probably the most common variant is ComponentVariant, which is a hierarchical structure consisting of nested GroupingComponents. Both variants and components identify themselves using a description attribute, which variants base on their type and components base on their id.

A much less common variant is the FallbackVariant, which we use when we don't have enough data for any of the other grouping methods to work. Perhaps its rarity (combined with the infrequency with which people pay close attention to grouping info data) explains why we've never noticed this, but fallback variants have an id (as if they were components) rather than a type (as they should as variants). As a result, their description is None rather than a real, useful value.

This fixes that by switching the attribute name from id to type.