subscriber: fix max_level_hint for empty Option/Vec Layer impls (tokio-rs#2195)

## Motivation

These are incorrect: currently, when you have a `None` layer, the `None`
hint it returns causes the default `TRACE` to win, which is inaccurate.
Similarly, `Vec` should default to `OFF` if it has no `Layer`s in it

## Solution

Change the default hints to `Some(OFF)`

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

Author: 2 people

tracing-subscriber/src/layer/mod.rs

@@ -1490,7 +1490,11 @@ where
     fn max_level_hint(&self) -> Option<LevelFilter> {
         match self {
             Some(ref inner) => inner.max_level_hint(),
-            None => None,
+            None => {
+                // There is no inner layer, so this layer will
+                // never enable anything.
+                Some(LevelFilter::OFF)
+            }
         }
     }
 
@@ -1701,7 +1705,8 @@ feature! {
         }
 
         fn max_level_hint(&self) -> Option<LevelFilter> {
-            let mut max_level = LevelFilter::ERROR;
+            // Default to `OFF` if there are no inner layers.
+            let mut max_level = LevelFilter::OFF;
             for l in self {
                 // NOTE(eliza): this is slightly subtle: if *any* layer
                 // returns `None`, we have to return `None`, assuming there is

tracing-subscriber/tests/layer_filters/vec.rs

@@ -111,3 +111,10 @@ fn all_filtered_max_level_hint() {
 
     assert_eq!(subscriber.max_level_hint(), Some(LevelFilter::DEBUG));
 }
+
+#[test]
+fn empty_vec() {
+    // Just a None means everything is off
+    let subscriber = tracing_subscriber::registry().with(Vec::<ExpectLayer>::new());
+    assert_eq!(subscriber.max_level_hint(), Some(LevelFilter::OFF));
+}