Attach (recommended) or Link to PDF file here:
[PDFBOX-5275-p5.pdf](https://github.com/mozilla/pdf.js/files/7109095/PDFBOX-5275-p5.pdf)

Configuration:
- Web browser and its version: FF 91.0.2
- Operating system and its version: Windows 10.0.19043.1165
- PDF.js version: latest (tried with https://mozilla.github.io/pdf.js/web/viewer.html )

Steps to reproduce the problem:
1. open the file
2.

What is the expected behavior? Displays a plant

What went wrong? Displays blank page

See also
https://issues.apache.org/jira/browse/PDFBOX-5275
which had the same problem, the cause was bad handling of an empty OCGs entry

![grafik](https://user-images.githubusercontent.com/6665575/132079956-c48921bf-6dbb-4e89-886e-4301d955ee35.png)


const length = array.length;
    if (length < 2) {
      return true;
    }
    const operator = array[0];
    for (let i = 1; i < length; i++) {
      const element = array[i];
      let state;
      if (Array.isArray(element)) {
        state = this._evaluateVisibilityExpression(element);
      } else if (this._groups.has(element)) {
        state = this._groups.get(element).visible;
      } else {
        warn(`Optional content group not found: ${element}`);