Fix: GPv3: Clear copy strokes buffer if nothing is selected during copy

If `clipboard.curves` has some curve and the copy operation is called
again when nothing is selected, the buffer is expected to be cleared.
Otherwise the paste operations will paste the previous copied curves.

Pull Request: https://projects.blender.org/blender/blender/pulls/120454

source/blender/editors/grease_pencil/intern/grease_pencil_edit.cc

@@ -2354,6 +2354,7 @@ static int grease_pencil_copy_strokes_exec(bContext *C, wmOperator *op)
   }
 
   if (!anything_copied) {
+    clipboard.curves.resize(0, 0);
     return OPERATOR_CANCELLED;
   }