Instant Search: allow default results to be turned off

#### Is your feature request related to a problem? Please describe.
When you open the Jetpack Search modal without a query, results are still shown. If sorting by relevance, these will be the posts and pages deemed the most relevant across the whole site. The idea is that the user can start with all results and then use filters to find what they want.


A user has reported that the first two pages shown on their default results are very old posts and are not especially relevant. They would like the ability to turn off the default results when a search query has not been entered.

#### Describe the solution you'd like
The user is able to turn off the default results using Jetpack Search settings.

We should also consider making this the default option for new users.

#### Describe alternatives you've considered
We could improve the relevance search. In this case, reducing the relevance score of older results would have helped somewhat.

#### Additional context
Slack conversation: p1615844037002700-slack-CLKR331CM

The issue identifies the Jetpack Search modal and Jetpack Search settings as the relevant entry points, but names no files or tests. Start by locating the modal's empty-query behavior and the settings that control Search results. Done means a setting can disable default results, with the new-user default clarified and covered by the project's existing tests.

Written by the indexing model from the issue text.