Reset property in toolbar for each script run

Question

I would like to reset the string property in the toolbar every time I hit the "Object" in the Add.

In short, this is the code I'm using:

class primitive_object_add(bpy.types.Operator):
    bl_idname = "mesh.primitive_object_add"
    bl_label = "Add an object"
    bl_options = {'REGISTER', 'UNDO'}
    img_str = StringProperty(name="Image Name (example: name.png)", description="desc here", default="", subtype='FILE_PATH')

    def execute(self, context):
        #code here

def menu_func(self, context):
    self.layout.operator("mesh.primitive_object_add", text="Object", icon='OBJECT_DATA')


def register():
    bpy.utils.register_module(__name__)
    bpy.types.INFO_MT_mesh_add.append(menu_func)

def unregister():
    bpy.utils.unregister_module(__name__)
    bpy.types.INFO_MT_mesh_add.remove(menu_func)

if __name__ == "__main__":
    register()

I've tried using self.img_str = "" However if I'm not fully sure where I could put this in the code to have it run when I press "Object"