Many missing returns

Problem/Motivation

phpstan reports, among other things,

------ --------------------------------------------------------------------------------------------------------------
Line src/DisplayBase.php
------ --------------------------------------------------------------------------------------------------------------
167 Method Drupal\entity_browser\DisplayBase::displayEntityBrowser() should return array but return statement is
missing.
------ --------------------------------------------------------------------------------------------------------------

------ -----------------------------------------------------------------------------------------------------------
Line src/Form/EntityBrowserEditForm.php
------ -----------------------------------------------------------------------------------------------------------
464 Method Drupal\entity_browser\Form\EntityBrowserEditForm::save() should return int but return statement is
missing.
------ -----------------------------------------------------------------------------------------------------------

------ ------------------------------------------------------------------------------------------------------------
Line src/Plugin/EntityBrowser/Display/Standalone.php
------ ------------------------------------------------------------------------------------------------------------
50 Method Drupal\entity_browser\Plugin\EntityBrowser\Display\Standalone::displayEntityBrowser() should return
array but return statement is missing.
------ ------------------------------------------------------------------------------------------------------------

------ ----------------------------------------------------------------------------------------------------------
Line src/Plugin/EntityBrowser/WidgetSelector/Tabs.php
------ ----------------------------------------------------------------------------------------------------------
52 Method Drupal\entity_browser\Plugin\EntityBrowser\WidgetSelector\Tabs::submit() should return string but
return statement is missing.
------ ----------------------------------------------------------------------------------------------------------

------ ---------------------------------------------------------------------------------------------------------
Line src/SelectionDisplayBase.php
------ ---------------------------------------------------------------------------------------------------------
116 Method Drupal\entity_browser\SelectionDisplayBase::label() should return string but return statement is
missing.
------ ---------------------------------------------------------------------------------------------------------

------ --------------------------------------------------------------------------------------------------------
Line src/WidgetSelectorBase.php
------ --------------------------------------------------------------------------------------------------------
109 Method Drupal\entity_browser\WidgetSelectorBase::submit() should return string but return statement is
missing.
------ --------------------------------------------------------------------------------------------------------

------ ---------------------------------------------------------------------------------------------------------
Line src/WidgetValidationBase.php
------ ---------------------------------------------------------------------------------------------------------
65 Method Drupal\entity_browser\WidgetValidationBase::label() should return string but return statement is
missing.
------ ---------------------------------------------------------------------------------------------------------

------ ---------------------------------------------------------------------------------------------------------
Line tests/modules/entity_browser_test/src/Plugin/EntityBrowser/Widget/PagerTestWidget.php
------ ---------------------------------------------------------------------------------------------------------
77 Method Drupal\entity_browser_test\Plugin\EntityBrowser\Widget\PagerTestWidget::prepareEntities() should
return array but return statement is missing.
------ ---------------------------------------------------------------------------------------------------------

Steps to reproduce

phpstan -l0 entity_browser

Proposed resolution

Add return statements.

Remaining tasks

Make an MR.