Avoid type error when the entity does not have any a view mode

Problem/Motivation

The visual inline diff creates an empty link operation when the entity type does not have a view mode declared.

The following exception is thrown:

TypeError: Unsupported operand types: null + array in Drupal\Core\Theme\ThemePreprocess->preprocessLinks() (line 157 of core/lib/Drupal/Core/Theme/ThemePreprocess.php).
Drupal\Core\Theme\ThemeManager->render() (Line: 500)
Drupal\Core\Render\Renderer->doRender() (Line: 513)
Drupal\Core\Render\Renderer->doRender() (Line: 513)
Drupal\Core\Render\Renderer->doRender() (Line: 513)
Drupal\Core\Render\Renderer->doRender() (Line: 230)
Drupal\Core\Render\Renderer->render() (Line: 242)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 634)
Drupal\Core\Render\Renderer::Drupal\Core\Render\{closure}()
Fiber->start() (Line: 635)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 235)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare() (Line: 131)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse() (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray() (Line: 246)
Symfony\Component\EventDispatcher\EventDispatcher::Symfony\Component\EventDispatcher\{closure}() (Line: 206)
Symfony\Component\EventDispatcher\EventDispatcher->callListeners() (Line: 56)
Symfony\Component\EventDispatcher\EventDispatcher->dispatch() (Line: 188)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 53)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle() (Line: 118)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 92)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 53)
Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 54)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 745)
Drupal\Core\DrupalKernel->handle() (Line: 19)

Steps to reproduce

Attempt to view a diff for an entity that does not have a view mode.

Proposed resolution

Check that a view mode exists before unshifting a value into the array.

Remaining tasks

Provide MR.

User interface changes

N/A