Fix nginx template: guard proxy pass for initial/clean deployments

@VinneyJ Production deploy dispatch simulation works now for an app that was never deployed before. This is a separate nginx issue that will be addressed. However, since sensors prod has been deployed before and nginx configurations exist, the workflow will dispatch without any issues.

_Originally posted by @gideonmaina in https://github.com/CodeForAfrica/sensors.AFRICA-api/issues/165#issuecomment-5325762043_

Start by locating the nginx template used by the deployment workflow and compare its behavior for an initial or clean deployment with one that already has nginx configuration. Verify that the production dispatch succeeds in the clean case without breaking deployments where configurations already exist.

Written by the indexing model from the issue text.