Handle nil in ConnectionPool::WeakThreadKeyMap

Author: byroot

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -133,7 +133,7 @@ def [](key)
         end
 
         def []=(key, value)
-          @map.select! { |c, _| c.alive? }
+          @map.select! { |c, _| c&.alive? }
           @map[key] = value
         end
       end