[joiner] refresh ID from EUI-64 in Start() when discerner is empty (#12393)

This commit updates `Joiner::Start()` to explicitly call
`SetIdFromIeeeEui64()` when `mDiscerner` is empty. This ensures that
the Joiner ID is freshly derived from the current IEEE EUI-64 address,
guaranteeing correctness even if the address was not ready during
initialization or has changed since the instance was created.

src/core/meshcop/joiner.cpp

@@ -128,6 +128,11 @@ Error Joiner::Start(const char      *aPskd,
 
     SuccessOrExit(error = Get<Tmf::SecureAgent>().Open(Ip6::NetifIdentifier::kNetifThreadInternal));
 
+    if (mDiscerner.IsEmpty())
+    {
+        SetIdFromIeeeEui64();
+    }
+
     // After this, if any of the steps fails, we need to cleanup
     // (free allocated message, stop seeker, close agent, etc).
     shouldCleanup = true;