Call default onDrop() only when there are non-user-scripts.
Call default onDrop() only when there are non-user-scripts.

Fixes #1663

Martii/greasemonkey,the8472/greasemonkey,HasClass0/greasemonkey,HasClass0/greasemonkey,the8472/greasemonkey,Ventero/greasemonkey,Ventero/greasemonkey,Martii/greasemonkey