macros: Fix non-simple macro usage without message (#2879)

If something like `warn!(?foo)` and `warn!(name: "foo", ?foo)` works, then `warn!(name: "foo", target: "foo_events", ?foo)` should arguably also work. Before this change the more complicated variants of the macros however required a message argument, due to the usage of the `+` specifier in the macro definitions. This commit changes the `+` (1 or more) to `*` (0 or more), which matches how the `field` tokens are used in the simpler variants of the macro.