Add a test that would have caught the bug last week (#38222)

This adds a test to make sure that the default value of the auto update
setting is always true. We manually re-applied the broken code from last
week, and confirmed that this test fails with that code.

Release Notes:

- N/A

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/project/src/direnv.rs

@@ -65,7 +65,7 @@ pub async fn load_direnv_environment(
     let output = String::from_utf8_lossy(&direnv_output.stdout);
     if output.is_empty() {
         // direnv outputs nothing when it has no changes to apply to environment variables
-        return Ok(HashMap::new());
+        return Ok(HashMap::default());
     }
 
     match serde_json::from_str(&output) {