Don't warn when dismissing two layers of tree nav (pointfreeco#3037)

Currently, popping two layers in a nav tree will cause intermediate,
invalid stores to send `dismiss` actions when they shouldn't. This
branch avoids this by checking the store's validity before sending the
action along.

Fixes pointfreeco#3031.

Author: stephencelis

Sources/ComposableArchitecture/Observation/Store+Observation.swift

@@ -292,7 +292,7 @@
         #endif
       }
       set {
-        if newValue == nil, self.state[keyPath: state] != nil {
+        if newValue == nil, self.state[keyPath: state] != nil, !self._isInvalidated() {
           self.send(action(.dismiss))
         }
       }