Template: add sanity check on output buffering
Template: add sanity check on output buffering

Fixes PHPStan warning:

> Parameter #1 $str of method Smr\Template::convertHtmlToAjaxXml()
> expects string, string|false given.

If output buffering is not turned on, then the template will not be
able to display anything.

smrealms/smr,hemberger/smr,hemberger/smr,hemberger/smr,smrealms/smr,smrealms/smr,hemberger/smr