Block build returns NULL, causing fatal error with Drupal Canvas

Problem/motivation

This module's plugin block will return a NULL value under certain circumstances. Per the plugin block contract, the build() method needs to return a renderable array. The failure to return a renderable array results in triggering a fatal error when this module is enabled along with Drupal Canvas. See #3570822: Canvas shows error when trying to build page through Canvas.

Steps to reproduce

1. Install this module and Drupal Canvas.
2. Create a new Canvas page and click on the "+" icon to add a block
3. Observe error "Error 500: Drupal\Core\Render\Element::isEmpty(): Argument #1 ($elements) must be of type array, null given"

Proposed resolution

Return an empty array rather than a NULL value in $block->build()