Top Posts and Goodreads blocks: users able to disable all display settings

Somewhat of a nitpick, but users are able to turn off all "Items to display" for `top-posts` block, and all of the "Display settings" for the `goodreads` block, such that these blocks wouldn't render anything useful to display. Perhaps keep one default toggle enabled when all others are turned off.

Via Jetpack 13.2 internal testing: p8oabR-1t3-p2#comment-7895

Locate the implementations and display-setting controls for the top-posts and goodreads blocks, then reproduce the case where every option is disabled. Done should mean each block retains one enabled default or otherwise avoids rendering without useful content; check the existing block behavior and any relevant tests before changing both cases.

Written by the indexing model from the issue text.