fix sign in with custom token api don't return local id

Author: anotherdev

firebase-auth-rest/core/src/main/java/com/anotherdev/firebase/auth/SignInResponse.java

@@ -25,6 +25,7 @@ public interface SignInResponse {
     @SerializedName("expiresIn")
     String getExpiresIn();
 
+    @Nullable
     @SerializedName("localId")
     String getLocalId();