Add extra check in case super 'treeForVendor()' returns nothing
Add extra check in case super 'treeForVendor()' returns nothing

When merging trees, first use the super `treeForVendor()` tree (when
available), or use the passed in tree (when available), or else just
return the froala tree.

Panman8201/ember-froala-editor,froala/ember-froala-editor,froala/ember-froala-editor,Panman8201/ember-froala-editor