fix(render): resize error when item undefined

[wggit2020]

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of the default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

Steps:
1 Get a dom that has no<img> labels to new Viewer().
For example: change the code of 'docs/coutom-title.html'

<div id="galley">
<!-- remove children dom -->
</div>

window.addEventListener('DOMContentLoaded', function () {
    var galley = document.getElementById('galley');
    var viewer = new Viewer(galley, {
        url: 'data-original',
        title: function (image) {
          return image.alt + ' (' + (this.index + 1) + '/' + this.length + ')';
        },
    });
    // only add this code
    viewer.show();
});

2 Browse the 'coutom-title.html' in browser, resize the window, you will show the error console in DevTools.

// stack
viewer.js:1158 
    Uncaught TypeError: Cannot read properties of undefined (reading 'nextElementSibling')
    at Viewer.renderList (viewer.js:1158)
    at Viewer.resize (viewer.js:1748)