Restore Java 8 support in encoders lib (firebase#1627)

Author: mrwillis21

encoders/firebase-encoders-processor/src/main/java/com/google/firebase/encoders/processor/getters/GetterFactory.java

@@ -85,7 +85,7 @@ private Optional<Getter> create(DeclaredType ownerType, ExecutableElement elemen
  return Optional.empty();
  }
  Optional<String> fieldName = inferName(element);
- if (fieldName.isEmpty()) {
+ if (!fieldName.isPresent()) {
  return Optional.empty();
  }
  TypeMirror returnType = resolveTypeArguments(ownerType, element.getReturnType());