openrtb-core/src/main/java/com/google/openrtb/json/AbstractOpenRtbJsonReader.java
protected final boolean emptyToNull(JsonParser par) throws IOException {
    JsonToken token = par.getCurrentToken();
    if (token == null) {
      token = par.nextToken();
    }
    return !factory().isStrict() && token == null;
  }
protected final boolean emptyToNull(JsonParser par) throws IOException {
    JsonToken token = par.getCurrentToken();
    if (token == null) {
      token = par.nextToken();
    }
    return !factory().isStrict() && token == null;
  }
Special case for empty-string input. Returning null in non-@Nullable method,
but this is non-strict mode anyway.