subscriber: <None as Subscribe>::event_enabled should be true (#2193

)

## Motivation

This is wrong.

## Solution

Make it unwrong.

As described by the documentation on `Subscribe::event_enabled`, the
return value sets the global filtering of an event. This code used to
say that that `Option::<impl Subscribe>::None` existing in the
subscriber stack should disable any subscriber in the stack seeing
events

tracing-subscriber/src/subscribe/mod.rs

@@ -1507,7 +1507,7 @@ where
     fn event_enabled(&self, event: &Event<'_>, ctx: Context<'_, C>) -> bool {
         match self {
             Some(ref inner) => inner.event_enabled(event, ctx),
-            None => false,
+            None => true,
         }
     }