Images do not have an aria-label in the xwiki-slash dropdown for CKEditor

Recently, we got a regression on WCAG tests. I suppose it's related to the cleanup in https://github.com/xwiki/xwiki-platform/commit/172721962f7b3adaa3dbdb0ae3df8714bddbef90 in here, one rule was removed that properly hid those empty img elements. We should hide those img elements again.

```
WCAG fails in the test class [org.xwiki.ckeditor.test.ui.AllIT]:
__________
Validation in the test method [paragraphQuickAction_en]
Check for [org.xwiki.test.ui.po.editor.WikiEditPage] at http://xwikiweb:8080/xwiki/bin/edit/NestedLocalizationIT/paragraphQuickAction_en/WebHome?form_token=SL73qJAAaHkfEyJ7g7x1Vg&editor=wysiwyg.
Found [1] items

1: Images must have alternative text
Description: Ensure <img> elements have alternative text or a role of none or presentation
Help URL: https://dequeuniversity.com/rules/axe/4.10/image-alt?application=axeAPI
Help: Images must have alternative text
Impact: critical
Tags: cat.text-alternatives, wcag2a, wcag111, section508, section508.22.a, TTv5, TT7.a, TT7.b, EN-301-549, EN-9.1.1.1, ACT

HTML element:
<img src="">
Selector: cke_141 > .ckeditor-autocomplete-item-head > .ckeditor-autocomplete-item-icon-wrapper > img[src=""]
Fix any of the following:
Element does not have an alt attribute
aria-label attribute does not exist or is empty
aria-labelledby attribute does not exist, references elements that do not exist or references elements that are empty
Element has no title attribute
Element's default semantics were not overridden with role="none" or role="presentation"

HTML element:
<img src="">
Selector: cke_142 > .ckeditor-autocomplete-item-head > .ckeditor-autocomplete-item-icon-wrapper > img[src=""]
Fix any of the following:
Element does not have an alt attribute
aria-label attribute does not exist or is empty
aria-labelledby attribute does not exist, references elements that do not exist or references elements that are empty
Element has no title attribute
Element's default semantics were not overridden with role="none" or role="presentation"

HTML element:
<img src="">
Selector: cke_143 > .ckeditor-autocomplete-item-head > .ckeditor-autocomplete-item-icon-wrapper > img[src=""]
Fix any of the following:
Element does not have an alt attribute
aria-label attribute does not exist or is empty
aria-labelledby attribute does not exist, references elements that do not exist or references elements that are empty
Element has no title attribute
Element's default semantics were not overridden with role="none" or role="presentation"
[...]

```

There's about 40 of those reports in a row total.