Allow configuring the trace source logger without a TraceListener ins…

src/Microsoft.Extensions.Logging.TraceSource/TraceSourceLoggerProvider.cs

@@ -20,6 +20,15 @@ public class TraceSourceLoggerProvider : ILoggerProvider
 
         private bool _disposed = false;
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="TraceSourceLoggerProvider"/> class.
+        /// </summary>
+        /// <param name="rootSourceSwitch"></param>
+        public TraceSourceLoggerProvider(SourceSwitch rootSourceSwitch)
+            : this(rootSourceSwitch, null)
+        {
+        }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="TraceSourceLoggerProvider"/> class.
         /// </summary>
@@ -32,11 +41,6 @@ public TraceSourceLoggerProvider(SourceSwitch rootSourceSwitch, TraceListener ro
                 throw new ArgumentNullException(nameof(rootSourceSwitch));
             }
 
-            if (rootTraceListener == null)
-            {
-                throw new ArgumentNullException(nameof(rootTraceListener));
-            }
-
             _rootSourceSwitch = rootSourceSwitch;
             _rootTraceListener = rootTraceListener;
         }
@@ -115,7 +119,10 @@ public void Dispose()
             {
                 _disposed = true;
                 _rootTraceListener.Flush();
-                _rootTraceListener.Dispose();
+                if (_rootTraceListener != null)
+                {
+                    _rootTraceListener.Dispose();
+                }
             }
         }
     }