Attach (recommended) or Link to PDF file here:
[FormDB2_fr.pdf](https://github.com/mozilla/pdf.js/files/7352426/FormDB2_fr.pdf)

Configuration:
- Web browser and its version: Firefox nightly
- Operating system and its version: linux debian

Steps to reproduce the problem:
1. Open the pdf
2. The image on the top of each pages is missing 

What is the expected behavior? (add screenshot)
![image](https://user-images.githubusercontent.com/5641725/137465500-e8788893-f33d-4532-871d-43b464fa85f4.png)

What went wrong? (add screenshot)
![image](https://user-images.githubusercontent.com/5641725/137465363-f2b35fe4-6776-45e9-b9e4-ccef6d9ad670.png)

if (this.exData) {
      if (typeof this.exData[$content] === "string") {
        return this.exData[$content].trim();
      }
      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 {