fix: ensure completion block is always called in `DefaultExperiment…

…Client.start` function

Sources/Experiment/ExperimentClient.swift

@@ -98,6 +98,7 @@ internal class DefaultExperimentClient : NSObject, ExperimentClient {
         runningLock.wait()
         if isRunning {
             runningLock.signal()
+            completion(nil)
             return
         }
         isRunning = true