Create valid empty option in tpl_actiondropdown

Fix PHP Notices: Reduce error log noise

While DokuWiki suppresses PHP Notices they are still a code smell and
should be fixed. This fixes some PHP Notices that occurred.

Some of these fixes could be refactored into nicer code once we move to
PHP 7 and get access to the `??` operator.

The HTML-spec[0] specifies:

> If the element has no label attribute and is not a child of a datalist element: Text that is not inter-element whitespace.

So a non-breaking space is valid solution.

[0] https://html.spec.whatwg.org/multipage/form-elements.html#the-option-element

Translation update (fr)

Translation update (sr)

Translation update (sr)

Translation update (sr)

Translation update (de-informal)

Translation update (de-informal)

Translation update (pt)

This makes sure the provided extension ID is a valid id before
initializing it in the extension manager.

No need to check those seperately. The correct order should be ensured
already.

Update html.php

$regex initialization added to clearify that it is passed by reference and filled from subroutines

When a page is marked as hidden (through the hidepages option) it should
not be indexed by search engines.

Translation update (eu)

Translation update (eu)

the new version of the validator is static. But it makes sense to use a
single point of entry anyway, so instead of adjusting the use of the new
class, prefer the function.