NetworkFlipperPlugin#<init>'s param 'formatters' Receives Null Value

Summary:
mark formatters nullsafe as stated by the error code:

```
07-23 16:21:27.445  4845  4845 E AndroidRuntime: Caused by: java.lang.IllegalArgumentException: NetworkFlipperPlugin#<init>'s param 'formatters' received null value, but is not marked as Nullable.
07-23 16:21:27.445  4845  4845 E AndroidRuntime: 	at com.facebook.flipper.plugins.network.NetworkFlipperPlugin.<init>(NetworkFlipperPlugin.java:1)
07-23 16:21:27.445  4845  4845 E AndroidRuntime: 	at com.facebook.flipper.plugins.network.NetworkFlipperPlugin.<init>(NetworkFlipperPlugin.java:27)
07-23 16:21:27.445  4845  4845 E AndroidRuntime: 	at com.instagram.appinitializer.devtools.FlipperInitializer.init(FlipperInitializer.java:96)
07-23 16:21:27.445  4845  4845 E AndroidRuntime: 	at com.instagram.appinitializer.devtools.DevToolInitializer.init(DevToolInitializer.kt:27)
```

Also add nullable to field set to null + method returning null explicitly

## Context

We are in the process of making the entire codebase Nullsafe. This diff relies on a runtime codemod to track when null is passed into a parameter that is not marked as Nullable in java. Based on runtime data, we can be 100% certain that this parameter needs to be Nullable

Reviewed By: nicholeic

Differential Revision: D60174408

fbshipit-source-id: 0e41badfa743445bb295af25da30faaffcdc5477

android/plugins/network/src/main/java/com/facebook/flipper/plugins/network/NetworkFlipperPlugin.java

@@ -20,14 +20,14 @@ public class NetworkFlipperPlugin extends BufferingFlipperPlugin implements Netw
   public static final String ID = "Network";
   private static final int MAX_BODY_SIZE_IN_BYTES = 1024 * 1024;
 
-  private List<NetworkResponseFormatter> mFormatters;
-  private final List<NetworkRequestFormatter> mRequestFormatters;
+  private @Nullable List<NetworkResponseFormatter> mFormatters;
+  private @Nullable final List<NetworkRequestFormatter> mRequestFormatters;
 
   public NetworkFlipperPlugin() {
     this(null);
   }
 
-  public NetworkFlipperPlugin(List<NetworkResponseFormatter> formatters) {
+  public NetworkFlipperPlugin(@Nullable List<NetworkResponseFormatter> formatters) {
     this.mFormatters = formatters;
     this.mRequestFormatters = null;
   }
@@ -171,7 +171,7 @@ protected void runOrThrow() throws Exception {
         .run();
   }
 
-  public static String toBase64(@Nullable byte[] bytes) {
+  public static @Nullable String toBase64(@Nullable byte[] bytes) {
     if (bytes == null) {
       return null;
     }