Don't crash when writing empty XMP collections.
Don't crash when writing empty XMP collections.

hwahrmann/taglib-sharp,Clancey/taglib-sharp,Clancey/taglib-sharp,archrival/taglib-sharp,punker76/taglib-sharp,mono/taglib-sharp,punker76/taglib-sharp,CamargoR/taglib-sharp,Clancey/taglib-sharp,hwahrmann/taglib-sharp,CamargoR/taglib-sharp,archrival/taglib-sharp