fix WritableArray, WritableMap nullable annotations (#23397)

Summary:
Recently, I added nullable annotations to ReadableArray, ReadableMap, WritableArray, WritableMap and subclasses to improve Kotlin developer experience. But found that I made mistake with pushArray, pushMap, pushString method of WritableArray, and putArray, putMap, putString methods of WritableMap. This PR fixes previous mistake.

Excerpt from WritableNativeArray.cpp.
```cpp
void WritableNativeArray::pushString(jstring value) {
  if (value == NULL) {
    pushNull();
    return;
  }
  throwIfConsumed();
  array_.push_back(wrap_alias(value)->toStdString());
}

void WritableNativeArray::pushNativeArray(WritableNativeArray* otherArray) {
  if (otherArray == NULL) {
    pushNull();
    return;
  }
  throwIfConsumed();
  array_.push_back(otherArray->consume());
}

void WritableNativeArray::pushNativeMap(WritableNativeMap* map) {
  if (map == NULL) {
    pushNull();
    return;
  }
  throwIfConsumed();
  array_.push_back(map->consume());
}
```

Excerpt from WritableNativeMap.cpp
```cpp
void WritableNativeMap::putString(std::string key, alias_ref<jstring> val) {
  if (!val) {
    putNull(std::move(key));
    return;
  }
  throwIfConsumed();
  map_.insert(std::move(key), val->toString());
}

void WritableNativeMap::putNativeArray(std::string key, WritableNativeArray* otherArray) {
  if (!otherArray) {
    putNull(std::move(key));
    return;
  }
  throwIfConsumed();
  map_.insert(key, otherArray->consume());
}

void WritableNativeMap::putNativeMap(std::string key, WritableNativeMap *otherMap) {
  if (!otherMap) {
    putNull(std::move(key));
    return;
  }
  throwIfConsumed();
  map_.insert(std::move(key), otherMap->consume());
}
```

[Android] [Changed] - fix nullable annotations in WritableArray, WritableMap
Pull Request resolved: https://github.com/facebook/react-native/pull/23397

Differential Revision: D14044014

Pulled By: cpojer

fbshipit-source-id: c44ea2e097e7b1156223b516aa640a181f0d4a9b

ReactAndroid/src/main/java/com/facebook/react/bridge/JavaOnlyArray.java

@@ -160,7 +160,7 @@ public class JavaOnlyArray implements ReadableArray, WritableArray {
   }
 
   @Override
-  public void pushString(@Nonnull String value) {
+  public void pushString(@Nullable String value) {
     mBackingList.add(value);
   }
 
@@ -170,7 +170,7 @@ public class JavaOnlyArray implements ReadableArray, WritableArray {
   }
 
   @Override
-  public void pushMap(@Nonnull WritableMap map) {
+  public void pushMap(@Nullable WritableMap map) {
     mBackingList.add(map);
   }