subscriber: <None as Layer>::event_enabled should be true (tokio-rs#2193)

## Motivation

This is wrong.

## Solution

Make it unwrong.

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

Author: CAD97

tracing-subscriber/src/layer/mod.rs

@@ -1512,7 +1512,7 @@ where
     fn event_enabled(&self, event: &Event<'_>, ctx: Context<'_, S>) -> bool {
         match self {
             Some(ref inner) => inner.event_enabled(event, ctx),
-            None => false,
+            None => true,
         }
     }