[BUG] @app.long_callback has interval set to None

[rymndhng]

I recently upgraded our dash code (from 2.5.1 to 2.8.0).

I noticed that our long callbacks using the original @app.long_callback decorator is polling our backend many times a second, instead of only once a second.

If I switch over to the new background=True, then the 1 second polling is restored.

I think this is a undesired client regression.

Describe your context
Please provide us your environment, so we can easily reproduce the issue.

• replace the result of pip list | grep dash below

dash                      2.8.0
dash-core-components      2.0.0
dash-html-components      2.0.0
dash-table                5.0.0

Describe the bug

My frontend application is polling the backend excessively using @app.long_callback when the interval value is unset.

Expected behavior

When a long callback is created using @app.long_callback, the default interval of 1 second should be used.