Deprecated function explode() with PHP 8.1

PHP 8 works a little bit in another way than PHP 7, and we should adapt this module to php8

Steps to reproduce

Just use BackgroundImage block effect

API changes

Add one more condition before call explode() function.

Error message:

Deprecated function: explode(): Passing null to parameter #2 ($string) of type string is deprecated in Drupal\block_generation\Plugin\BlockGenerationEffect\BackgroundImage->applyEffect() (line 128 of modules/contrib/block_generation/src/Plugin/BlockGenerationEffect/BackgroundImage.php).
Drupal\block_generation\Plugin\BlockGenerationEffect\BackgroundImage->applyEffect('wrapper_external', Array) (Line: 544)
_block_generation_process_effects('wrapper_external', Object, Array) (Line: 406)
_block_generation_process_wrapper('wrapper_external', Object, Array) (Line: 265)
template_preprocess_block__block_generation(Array, 'block__block_generation__block_generation_teaser', Array)
call_user_func_array('template_preprocess_block__block_generation', Array) (Line: 349)
Drupal\Core\Theme\ThemeManager->render('block__block_generation__block_generation_teaser', Array) (Line: 422)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 201)
Drupal\Core\Render\Renderer->render(Array) (Line: 479)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1) (Line: 158)
__TwigTemplate_36b0a742301cb3b92f333aab3b1f95c2->block_content(Array, Array) (Line: 182)
Twig\Template->displayBlock('content', Array, Array) (Line: 112)
__TwigTemplate_36b0a742301cb3b92f333aab3b1f95c2->block_container(Array, Array) (Line: 182)
Twig\Template->displayBlock('container', Array, Array) (Line: 89)
__TwigTemplate_36b0a742301cb3b92f333aab3b1f95c2->doDisplay(Array, Array) (Line: 405)
Twig\Template->displayWithErrorHandling(Array, Array) (Line: 378)
Twig\Template->display(Array) (Line: 390)
Twig\Template->render(Array) (Line: 70)
twig_render_template('themes/contrib/bootstrapper/templates/system/block.html.twig', Array, Array) (Line: 470)
Drupal\Core\Theme\ThemeManager->render('block', Array) (Line: 422)
Drupal\Core\Render\Renderer->doRender(Array) (Line: 435)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 201)
Drupal\Core\Render\Renderer->render(Array) (Line: 479)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1) (Line: 127)
__TwigTemplate_c1f83ca509e6a7d87faba56438ff1627->doDisplay(Array, Array) (Line: 405)
Twig\Template->displayWithErrorHandling(Array, Array) (Line: 378)
Twig\Template->display(Array) (Line: 390)
Twig\Template->render(Array) (Line: 70)
twig_render_template('modules/contrib/grids/layouts/onecol/layout--grids-onecol.html.twig', Array, Array) (Line: 470)
Drupal\Core\Theme\ThemeManager->render('layout__grids_onecol', Array) (Line: 422)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 201)
Drupal\Core\Render\Renderer->render(Array, ) (Line: 241)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 564)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 242)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object, Object) (Line: 132)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object) (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object)
call_user_func(Array, Object, 'kernel.view', Object) (Line: 142)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object, 'kernel.view') (Line: 163)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 191)
Drupal\page_cache\StackMiddleware\PageCache->fetch(Object, 1, 1) (Line: 128)
Drupal\page_cache\StackMiddleware\PageCache->lookup(Object, 1, 1) (Line: 82)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 50)
Drupal\ban\BanMiddleware->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 49)
Drupal\remove_http_headers\StackMiddleware\RemoveHttpHeadersMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 702)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)