fix sd example returning bad json on empty folder

Author: John Doe

hardware/esp8266com/esp8266/libraries/ESP8266WebServer/examples/SDWebServer/SDWebServer.ino

@@ -183,16 +183,15 @@ void printDirectory() {
   server.setContentLength(CONTENT_LENGTH_UNKNOWN);
   server.send(200, "text/json", "");
   WiFiClient client = server.client();
-
+  
+  server.sendContent("[");
   for (int cnt = 0; true; ++cnt) {
     File entry = dir.openNextFile();
     if (!entry)
     break;
 
     String output;
-    if (cnt == 0) 
-      output = '[';
-    else 
+    if (cnt > 0)
       output = ',';
 
     output += "{\"type\":\"";