[layout] Fix html item freeze QGIS when loading HTML undet Qt6/QtWebkit-less environments

[nirvn]

Description

This PR fixes a QGIS - built against Qt6 (or when WITH_QTWEBKIT=FALSE - freezes when a layout HTML item loads its content with an atlas context defined.

When QtWebkit is not present, calling mWebPage->mainFrame()->evaluateJavaScript() does nothing. In turn, it means that the JavascriptExecutorLoop jsLoop object never has its done() function called, leading to QGIS freezing for eternity.

[3nids]

should we inject some kind of message to state that it's not being loaded?

[nirvn]

@3nids , I don't think an additional warning here is warranted here per say. The item isn't "lesser" than before, in that without QtWebkit, the item would simply never render full stop. With an atlas, it was not rendering and freezing :)