Attach (recommended) or Link to PDF file here:
issue13500 (in test corpus)

This regressed when landing #13566. 

What is the expected behavior? (add screenshot)
<img width="664" alt="image" src="https://user-images.githubusercontent.com/942640/122467522-7bc8aa80-cf6f-11eb-9d4d-311eb7c64fb2.png">


What went wrong? (add screenshot)
![image](https://user-images.githubusercontent.com/942640/122467565-85eaa900-cf6f-11eb-9c10-8a8686ed440c.png)


if (this.exData) {
      return this.exData[$content][$text]().trim();
    }
    for (const name of Object.getOwnPropertyNames(this)) {
      if (name === "image") {
        continue;
      }
      const obj = this[name];
      if (obj instanceof XFAObject) {
        return (obj[$content] || "").toString().trim();
      }
    }
    return null;
  }
  [$toHTML](availableSpace) {
    for (const name of Object.getOwnPropertyNames(this)) {
      const obj = this[name];
      if (!(obj instanceof XFAObject)) {
        continue;
      }
      return obj[$toHTML](availableSpace);
    }
    return HTMLResult.EMPTY;
  }
}
class Variables extends XFAObject {