Avoid using ByteVector::null where an empty vector is required.

ByteVector::null is not necessarily be empty or remains the same instance.
Using it in a public header may lead to a linkage error.

taglib/mpeg/id3v2/id3v2frame.cpp

@@ -170,7 +170,7 @@ ByteVector Frame::frameID() const
   if(d->header)
     return d->header->frameID();
   else
-    return ByteVector::null;
+    return ByteVector();
 }
 
 TagLib::uint Frame::size() const