Warning: Undefined array key "#markup" in draggableviews_preprocess_views_view_table()

Problem/Motivation

When draggable views page doesn't have any item listed that time we are getting the warning message:

Warning: Undefined array key "#markup" in draggableviews_preprocess_views_view_table() (line 107 of modules/contrib/draggableviews/draggableviews.module).
draggableviews_preprocess_views_view_table(Array, 'views_view_table', Array)
call_user_func_array('draggableviews_preprocess_views_view_table', Array) (Line: 287)
Drupal\Core\Theme\ThemeManager->render('views_view_table', 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: 60)
Drupal\views\Form\ViewsFormMainForm::preRenderViewsForm(Array)
call_user_func_array(Array, Array) (Line: 101)
Drupal\Core\Render\Renderer->doTrustedCallback(Array, Array, 'Render #pre_render callbacks must be methods of a class that implements \Drupal\Core\Security\TrustedCallbackInterface or be an anonymous function. The callback was %s. See https://www.drupal.org/node/2966725 → ', 'exception', 'Drupal\Core\Render\Element\RenderCallbackInterface') (Line: 772)
Drupal\Core\Render\Renderer->doCallback('#pre_render', Array, Array) (Line: 363)
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: 110)
__TwigTemplate_df6517c700d507667af4ae461b2cf252f103b268b189fd55047a9f21a190524b->doDisplay(Array, Array) (Line: 405)
Twig\Template->displayWithErrorHandling(Array, Array) (Line: 378)
Twig\Template->display(Array) (Line: 390)
Twig\Template->render(Array) (Line: 55)
twig_render_template('core/themes/claro/templates/classy/views/views-view.html.twig', Array) (Line: 384)
Drupal\Core\Theme\ThemeManager->render('views_view', 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: 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: 174)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 81)
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: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 709)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

Remove all the element listed on draggable views page, when we don't have any item listed that time we will get this warning message.

Proposed resolution

Added the condition using Null coalescing operator to assign empty when markup doesn't have any value to return.