Python Dash dropdown在没有返回任何值后打开，我如何关闭它？

import dash
from dash.dependencies import Input, Output
import dash_core_components as dcc
import dash_html_components as html

app = dash.Dash(__name__)

app.layout = html.Div([
    dcc.Dropdown(
        id='my-dropdown',
        options=[
            {'label': 'Option 1', 'value': 'option1'},
            {'label': 'Option 2', 'value': 'option2'},
            {'label': 'Option 3', 'value': 'option3'}
        ],
        value=''
    ),
    html.Div(id='output')
])

@app.callback(
    Output('output', 'children'),
    [Input('my-dropdown', 'value')]
)
def update_output(value):
    if value is None:
        # 当dropdown的值为None时，关闭dropdown
        return dcc.Location(pathname='/')
    else:
        return 'You have selected "{}"'.format(value)