Don't panic when getting the ja4

Use the correct error so we return None.

Author: fgsch

lib/src/wiggle_abi/req_impl.rs

@@ -304,7 +304,7 @@ impl FastlyHttpReq for Session {
         ja4_max_len: u32,
         nwritten_out: GuestPtr<u32>,
     ) -> Result<(), Error> {
-        Err(Error::NotAvailable("Client TLS JA4 hash"))
+        Err(Error::ValueAbsent)
     }
 
     fn downstream_compliance_region(

test-fixtures/src/bin/downstream-req.rs

@@ -33,6 +33,8 @@ fn main() {
     assert_eq!(client_req.get_tls_client_hello(), None);
     assert_eq!(client_req.get_tls_protocol(), None);
     assert_eq!(client_req.get_tls_protocol_bytes(), None);
+    assert_eq!(client_req.get_tls_ja3_md5(), None);
+    assert_eq!(client_req.get_tls_ja4(), None);
     // NOTE: This currently fails, waiting on a patch to land in the fastly crate
     // assert_eq!(client_req.get_tls_raw_client_certificate(), None);
     assert_eq!(client_req.get_tls_raw_client_certificate_bytes(), None);