#5777 fix ApolloClient.Builder.okHttpClient() returns null instead of…

… this (#5778)

(cherry picked from commit 807155d)

libraries/apollo-runtime-java/src/main/java/com/apollographql/apollo3/runtime/java/ApolloClient.java

@@ -225,7 +225,7 @@ public Builder webSocketServerUrl(@NotNull String webSocketServerUrl) {
     public Builder okHttpClient(@NotNull OkHttpClient okHttpClient) {
       this.callFactory = checkNotNull(okHttpClient, "okHttpClient is null");
       this.webSocketFactory = okHttpClient;
-      return null;
+      return this;
     }
 
     /**