[Bug]: Open Telemetry is configured when environment variables are blank

### What happened?

When starting the server with `chroma run /config.yaml`, Open Telemetry exporting is started if the following environment variables are set to blank values:

```
CHROMA_OPEN_TELEMETRY__ENDPOINT=""
CHROMA_OPEN_TELEMETRY__SERVICE_NAME=""
```

With the endpoint being blank, the following error occurs:
```
server-1 | thread 'main' panicked at rust/tracing/src/init_tracer.rs:69:10:
server-1 | could not build span exporter for tracing: ExportFailed(InvalidUri(InvalidUri(Empty)))
server-1 | note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

We'd expect that the variables being blank starts with open telemetry disabled the same way as if the variables are unset:

```
server-1 | OpenTelemetry is not enabled because it is missing from the config.
server-1 | Listening on 0.0.0.0:8000
```

### Versions

Chroma v1.0.10

### Relevant log output

```shell

```

Start in rust/tracing/src/init_tracer.rs around line 69 and inspect how CHROMA_OPEN_TELEMETRY__ENDPOINT and CHROMA_OPEN_TELEMETRY__SERVICE_NAME are interpreted. Reproduce startup with both variables set to empty strings, then verify it follows the existing disabled behavior for unset variables without panicking.

Written by the indexing model from the issue text.