Do not add empty CSS classes to the template

[fritzmg]

Q	A
Fixed issues	-
Docs PR or issue	-

Currently $classes will always be an array, passed down from ModuleArticle::compile. The array could contain first or last for example, but otherwise will be empty. In that case an empty string is added to the class template variable, resulting in HTML markup like this:

<div class="ce_example_element ">
  …
</div>

This PR adds a check for whether the $classes array is empty, to prevent the superfluous space.

[leofeyer]

We have to keep the is_array() check.

[leofeyer]

Thank you @fritzmg.