fix default parameters in ASSLineTagSection triggering an assertion when the section doesn't contain any tags
add removal of empty sections as cleaning level 1 to ASSLineContents:cleanTags(); fix broken processing in ASSLineTagSection:removeTags() if no tags are specified (it's supposed to delete all tags in range then but running self:callback and never returning anything because the "not tags" condition wasn't triggered actually caused tags to *modified* - might need to come back and check this later)