fix null parsing issue in request body

docs/examples/avatars/get-screenshot.md

@@ -0,0 +1,65 @@
+import 'package:appwrite/appwrite.dart';
+
+Client client = Client()
+    .setEndpoint('https://<REGION>.cloud.appwrite.io/v1') // Your API Endpoint
+    .setProject('<YOUR_PROJECT_ID>'); // Your project ID
+
+Avatars avatars = Avatars(client);
+
+// Downloading file
+UInt8List bytes = await avatars.getScreenshot(
+    url: 'https://example.com',
+    headers: {}, // optional
+    viewportWidth: 1, // optional
+    viewportHeight: 1, // optional
+    scale: 0.1, // optional
+    theme: .light, // optional
+    userAgent: '<USER_AGENT>', // optional
+    fullpage: false, // optional
+    locale: '<LOCALE>', // optional
+    timezone: .africaAbidjan, // optional
+    latitude: -90, // optional
+    longitude: -180, // optional
+    accuracy: 0, // optional
+    touch: false, // optional
+    permissions: [], // optional
+    sleep: 0, // optional
+    width: 0, // optional
+    height: 0, // optional
+    quality: -1, // optional
+    output: .jpg, // optional
+)
+
+final file = File('path_to_file/filename.ext');
+file.writeAsBytesSync(bytes);
+
+// Displaying image preview
+FutureBuilder(
+    future: avatars.getScreenshot(
+    url:'https://example.com' ,
+    headers:{} , // optional
+    viewportWidth:1 , // optional
+    viewportHeight:1 , // optional
+    scale:0.1 , // optional
+    theme: .light, // optional
+    userAgent:'<USER_AGENT>' , // optional
+    fullpage:false , // optional
+    locale:'<LOCALE>' , // optional
+    timezone: .africaAbidjan, // optional
+    latitude:-90 , // optional
+    longitude:-180 , // optional
+    accuracy:0 , // optional
+    touch:false , // optional
+    permissions:[] , // optional
+    sleep:0 , // optional
+    width:0 , // optional
+    height:0 , // optional
+    quality:-1 , // optional
+    output: .jpg, // optional
+), // Works for both public file and private file, for private files you need to be logged in
+    builder: (context, snapshot) {
+      return snapshot.hasData && snapshot.data != null
+          ? Image.memory(snapshot.data)
+          : CircularProgressIndicator();
+    }
+);