Allow component variants without twig template

It is already possible to define a component variant without a twig template. It is shown and processed correctly in storybook and also loaded correctly by drupal when embeding like '[COMPONENT]--[VARIANT]'. So far so good.

Unfortunately this results in a warning:
Warning: file_get_contents(themes/custom/scp/templates/components/molecules/menu/menu--header-bar.twig): Failed to open stream: No such file or directory in Drupal\cl_components\Component\ComponentMetadata->detectNamedBlocks() (line 314 of modules/contrib/cl_components/src/Component/ComponentMetadata.php).

The Provided MR falls back to the "default" variant when no template is available, which hopefully does not break any other integration and does like it meant to be.