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>