[dagit] Fix phantom overlay in playground
On the Playground, the "loading" overlay is rendered whether anything is loading or not. My recent change to pointer-events now prevents clicks from making it through to the controls underneath.
Instead, just don't render the loading overlay if it's not loading.
Test Plan: View Playground, verify that I can click on the "Launch Execution" button.
Reviewers: bengotow, sashank, prha
Reviewed By: sashank
Differential Revision: https://dagster.phacility.com/D5388