Only set up exporters if providers are properly set (#530)

This PR fixes the issue where DBOS would throw an exception if
`enable_otlp` is set to `True` but the logger/tracer providers weren't
set. Instead of crashing the entire app, it'll log a warning after this
change.

Also fixed an issue where `DBOS.trace` being `None` (when OTLP tracer
wasn't set) could crash the FastAPI app. It should only set the status
code if it can find an active span.