Implement lock aquiring shenanigans and restore related test

Thanks to armanbilge for the [suggestion](typelevel#4267 (comment))

Author: morgen-peschke

std/shared/src/main/scala/cats/effect/std/Hotswap.scala

@@ -17,6 +17,7 @@
 package cats.effect.std
 
 import cats.effect.kernel.{Concurrent, Resource}
+import cats.effect.kernel.Resource.ExitCase.Succeeded
 import cats.syntax.all._
 
 /**
@@ -106,7 +107,13 @@ object Hotswap {
         override def swap(next: Resource[F, R]): F[R] =
           nes.swap(next.map(_.some)) *> get.use(_.get.pure[F])
 
-        override def get: Resource[F, Option[R]] = nes.get
+        override def get: Resource[F, Option[R]] =
+          Resource.applyFull[F, Option[R]] { poll =>
+            poll(nes.get.allocatedCase).flatMap {
+              case (None, fin) => fin(Succeeded) *> F.pure((None, _ => F.unit))
+              case (r, fin) => F.pure((r, fin))
+            }
+          }
 
         override def clear: F[Unit] = nes.clear
       }

tests/shared/src/test/scala/cats/effect/std/HotswapSpec.scala

@@ -165,5 +165,15 @@ class HotswapSpec extends BaseSpec { outer =>
 
       TestControl.executeEmbed(go, IORuntimeConfig(1, 2)).replicateA_(1000) must completeAs(())
     }
+
+    "get should not acquire a lock when there is no resource present" in ticked {
+      implicit ticker =>
+        val go = Hotswap.create[IO, Unit].use { hs =>
+          hs.get.useForever.start *>
+            IO.sleep(2.seconds) *>
+            hs.swap(Resource.unit)
+        }
+        go must completeAs(())
+    }
   }
 }