Handle ConnectionProvider creation failure

Author: PopFlamingo

Sources/AsyncHTTPClient/ConnectionPool.swift

@@ -81,7 +81,7 @@ class ConnectionPool {
                     case .success(let provider):
                         self[key] = provider
                     case .failure:
-                        break
+                        self[key] = nil
                     }
                 }
                 return .make(promise)