Don't pass nullable strings to native DOM methods that want non-nullable strings. Fixes #1207.

src/components/script/dom/htmlvideoelement.rs

@@ -51,11 +51,11 @@ impl HTMLVideoElement {
         0
     }
 
-    pub fn Poster(&self) -> Option<DOMString> {
-        None
+    pub fn Poster(&self) -> DOMString {
+        ~""
     }
 
-    pub fn SetPoster(&mut self, _poster: &Option<DOMString>) -> ErrorResult {
+    pub fn SetPoster(&mut self, _poster: &DOMString) -> ErrorResult {
         Ok(())
     }
 }