Instant Search: category filter when there's only one category

#### Is your feature request related to a problem? Please describe.
The deafult category for posts is "Uncategorized". Jetpack Instant Search currently surfaces this category as an option to filter on, even when it's the only category used:

#### Describe the solution you'd like
One solution would be to not offer the category filters in setup if there’s only one category.

Start by locating the Jetpack Instant Search setup code that builds category filters, then reproduce the case where “Uncategorized” is the only category. Determine how the filter options are generated and add coverage for a single-category site; done means no category filter is offered when only one category exists, while multiple categories still remain filterable.

Written by the indexing model from the issue text.