[ALOY-304] Special tag parsers are overwriting properties defined in TSS, even when empty

The current problem showed up in "platform_folders" test app. Basically, the special tag parser for the coverflow is overwriting the "images" property defined in the tss even though the special tag parsing is not used.

<CoverFlowView/>

I suspect this is the case with other parser as well, like Toolbar, ButtonBar, etc... I'll need to investigate to figure out which parsers are affected and then make sure they only overwrite the tss values when they actually contain values.