Insights Geo Region selector: show "(No Detected Region)" in dropdown

### Problem Statement

Geo region may be missing for a non-trivial amount of spans, depending on a number of factors. The current selector doesn't show a `(No Detected Region)` option, like we have e.g. in the DB module:


If a user is not aware of this, they might miss a significant amount of data by looking at specific regions.

For example, [it is dependent](https://github.com/getsentry/relay/blob/bdf502c1821d21af6eb696f98e84aa16151d8c3d/relay-server/src/services/processor/span/processing.rs#L466) on the `ip_address` being sent by the SDK, which depends on config (e.g. `send_default_pii`) and can be overwritten on the client (e.g. if the app asks the user to opt-in to collecting this info). It can also be turned off or scrubbed on the backend.

### Solution Brainstorm

Add a `(No Detected Region)` option in the dropdown to make users aware of this and be able to include/exclude it.

### Product Area

Insights

No file or test is named. Start by locating the Insights Geo region selector and compare it with the analogous DB module dropdown shown in the issue; done means users can include or exclude a `(No Detected Region)` option alongside detected regions.

Written by the indexing model from the issue text.