Jetpack: Fix authors widget 'all' checkbox (#40878)

The block-based legacy widget editor sends the instance data through the
`update()` function multiple times, for preview and again on save. This
widget doing `isset( $new_instance['all'] )` was therefore going from
unset to `false` on preview and then from `false` to `true` on save.

Instead let's do like other widgets with checkboxes do, and cast the
value to boolean when set.

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/12658870949

Upstream-Ref: Automattic/jetpack@d3eb144

CHANGELOG.md

@@ -7,6 +7,7 @@
 This is an alpha version! The changes listed here are not final.
 
 ### Bug fixes
+- Authors widget: Fix saving of unchecked "Display all authors" checkbox in the legacy widget editor.
 - Fix custom roles settings are not sticking for Jetpack Stats
 
 ### Other changes <!-- Non-user-facing changes go here. This section will not be copied to readme.txt. -->

modules/widgets/authors.php

@@ -289,7 +289,7 @@ public function form( $instance ) {
 	 */
 	public function update( $new_instance, $old_instance ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
 		$new_instance['title']       = wp_strip_all_tags( $new_instance['title'] );
-		$new_instance['all']         = isset( $new_instance['all'] );
+		$new_instance['all']         = isset( $new_instance['all'] ) ? (bool) $new_instance['all'] : false;
 		$new_instance['number']      = (int) $new_instance['number'];
 		$new_instance['avatar_size'] = (int) $new_instance['avatar_size'];