[BUG] no output callbacks doesn't trigger as initial callback.

If you have a callback with no output and an initial input, the initial callback will not fire.

Example:
```python
from dash import Dash, html, Input, callback

app = Dash()

app.layout = html.Div([
 html.Button("Start", id="start-btn", n_clicks=0),
])


@callback(
 Input("start-btn", "n_clicks"),
)
def update(n_clicks):
 print(f"Clicked {n_clicks}")
```
