event: restore Err() logging when ErrorStackMarshaler returns nil (#763)

Commit f6fbd33 added `case nil: return e` in Event.Err() so that when
ErrorStackMarshaler returns nil (the error has no stack trace), the
function returns early without logging the error itself.

This was a regression. The previous behaviour was to fall through the
switch and always call AnErr(ErrorFieldName, err), ensuring the error
message is logged even when no stack is attached.

Libraries such as elastic/ecs-logging-go-zerolog (and plain fmt.Errorf
errors) rely on the old behaviour: calling Stack().Err(err) should
always log the error field; the stack field is additional and optional.

Change the `nil` case from `return e` to a comment (fall-through) so
that AnErr is called unconditionally. Update the test that was written
to match the incorrect behaviour.

Fixes #762

Signed-off-by: alliasgher <alliasgher123@gmail.com>

event.go

@@ -466,7 +466,8 @@ func (e *Event) Err(err error) *Event {
 	if e.stack && ErrorStackMarshaler != nil {
 		switch m := ErrorStackMarshaler(err).(type) {
 		case nil:
-			return e
+			// ErrorStackMarshaler returned nil — the error has no stack trace to
+			// attach. Fall through and still log the error via AnErr below.
 		case LogObjectMarshaler:
 			e = e.Object(ErrorStackFieldName, m)
 		case error:

event_test.go

@@ -720,7 +720,13 @@ func TestEvent_ErrWithStackMarshalerNil(t *testing.T) {
 	log.Log().Stack().Err(err).Msg("test message")
 
 	got := buf.String()
-	want := `{"message":"test message"}` + "\n" // No fields because stack marshaler returned nil
+	// When ErrorStackMarshaler returns nil (no stack trace available for this
+	// error), Err() must still log the error value via AnErr.  Without a stack
+	// field, the output is the same as if Stack() had not been called.
+	// Regression test for https://github.com/rs/zerolog/issues/762:
+	// commit f6fbd33 introduced a `case nil: return e` branch that silently
+	// swallowed the error instead of falling through to AnErr.
+	want := `{"error":"test error","message":"test message"}` + "\n"
 	if got != want {
 		t.Errorf("Event.Err() with nil stack marshaler = %q, want %q", got, want)
 	}