Update CardBrowser.java

[Georgezxh]

Purpose / Description

Audible cards are shown as free space in Card browser, it looks like empty card.

Fixes

Fix the issue: ankidroid#8871

Approach

In the cardbrowser, the displayed text of audible file card was replaced by "", so it looks the same as empty. So we just need to use something to replace "".

How Has This Been Tested?

before:

after:

Checklist

Please, go through these checks before submitting the PR.

• You have not changed whitespace unnecessarily (it makes diffs hard to read)
• You have a descriptive commit message with a short title (first line, max 50 chars).
• Your code follows the style of the project (e.g. never omit braces in if statements)
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)