[skip ci] Fix Nullsafe FIXMEs for MultipartStreamReader.java (facebook#50056)

Summary:

Gone trough all the FIXMEs added in the previous diff by the nullsafe tool, marked the class as nullsafe and ensured no remaining violations.

Changelog: [Android][Fixed] Made MultipartStreamReader.java nullsafe

Reviewed By: mdvacca

Differential Revision: D71126393

Author: GijsWeterings

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/devsupport/MultipartStreamReader.java

@@ -7,14 +7,17 @@
 
 package com.facebook.react.devsupport;
 
+import com.facebook.infer.annotation.Nullsafe;
 import java.io.IOException;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 import okio.Buffer;
 import okio.BufferedSource;
 import okio.ByteString;
 
 /** Utility class to parse the body of a response of type multipart/mixed. */
+@Nullsafe(Nullsafe.Mode.LOCAL)
 class MultipartStreamReader {
   // Standard line separator for HTTP.
   private static final String CRLF = "\r\n";
@@ -60,8 +63,7 @@ private void emitChunk(Buffer chunk, boolean done, ChunkListener listener) throw
     ByteString marker = ByteString.encodeUtf8(CRLF + CRLF);
     long indexOfMarker = chunk.indexOf(marker);
     if (indexOfMarker == -1) {
-      // NULLSAFE_FIXME[Parameter Not Nullable]
-      listener.onChunkComplete(null, chunk, done);
+      listener.onChunkComplete(Collections.emptyMap(), chunk, done);
     } else {
       Buffer headers = new Buffer();
       Buffer body = new Buffer();

packages/react-native/ReactAndroid/src/test/java/com/facebook/react/devsupport/MultipartStreamReaderTest.kt

@@ -33,7 +33,7 @@ class MultipartStreamReaderTest {
 
     val callback: CallCountTrackingChunkCallback =
         object : CallCountTrackingChunkCallback() {
-          override fun onChunkComplete(headers: Map<String, String>?, body: Buffer, done: Boolean) {
+          override fun onChunkComplete(headers: Map<String, String>, body: Buffer, done: Boolean) {
             super.onChunkComplete(headers, body, done)
 
             assertThat(done).isTrue
@@ -68,7 +68,7 @@ class MultipartStreamReaderTest {
 
     val callback: CallCountTrackingChunkCallback =
         object : CallCountTrackingChunkCallback() {
-          override fun onChunkComplete(headers: Map<String, String>?, body: Buffer, done: Boolean) {
+          override fun onChunkComplete(headers: Map<String, String>, body: Buffer, done: Boolean) {
             super.onChunkComplete(headers, body, done)
 
             assertThat(done).isEqualTo(callCount == 3)
@@ -125,7 +125,7 @@ class MultipartStreamReaderTest {
     var callCount = 0
       private set
 
-    override fun onChunkComplete(headers: Map<String, String>?, body: Buffer, done: Boolean) {
+    override fun onChunkComplete(headers: Map<String, String>, body: Buffer, done: Boolean) {
       callCount++
     }