Return null scope if no loggers

Author: benaadams

src/Microsoft.Framework.Logging/Logger.cs

@@ -97,6 +97,10 @@ public bool IsEnabled(LogLevel logLevel)
         public IDisposable BeginScopeImpl(object state)
         {
             var loggers = _loggers;
+            if (loggers.Length == 0)
+            {
+                return null;
+            }
             var scope = new Scope(loggers.Length);
             List<Exception> exceptions = null;
             for (var index = 0; index != loggers.Length; index++)
@@ -198,4 +202,4 @@ internal void Add(IDisposable disposable)
             }
         }
     }
-}
+}