[BUG] Pipedream Source - can not input custom expression when the async prop failed

**Describe the bug**
When the user creates a new Pipedream source having an async prop, when that async prop fails to load, the UI doesn't show the dropdown for the user to input a custom expression.

For example [in this thread here](https://pipedream-users.slack.com/archives/CPTJYRY5A/p1699539294351769?thread_ts=1699531596.930189&cid=CPTJYRY5A), a user tried to create a MongoDB source, but their credentials don't have list collection permission so they couldn't load the collection async prop, but they still want to use custom expression to input the collection name manually

**Expected behavior**
The Pipedream create source UI should allow the user to input a custom expression in case the async prop failed