Replace failing empty asset export with Item

Exporting some nodes in Figma (for example fully transparent ones) will result in 1x1 pixel image when using the Figma application or will result in an error in Qt Bridge for Figma. However, Bridge will export it, despite the lack of the asset, what is clearly wrong.

What we could do:

• Check for some conditions that lead to this (has no fill, is transparent, etc.)
• OR do try catch
• eventually replace such nodes with QML Item.