fix(search): change conditions for showing trailing button

[Antreesy]

☑️ Resolves

• Fix issue, described in *Close sidebar* focused and visible after page load on mobile #10908

when focusing the search box, we're facing a loop for mobiles / desktops with small screen resolution:

• Tabbing from input moves the focus to the trailing button
• At the nextTick, trailing button will be deleted, as system assumes search field to be blurred
• No element to focus => focus-trap falls back to the first available element => input field again

Workaround from Marco: setting a negative tabindex on the trailing button unless there is a non-empty value

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before

loop.mp4

🏡 After

not-loop.mp4

🏁 Checklist

• 🌏 Tested with Chrome, Firefox and Safari or should not be risky to browser differences
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required

[marcoambrosini]

@Antreesy see #10908 (comment)

[Antreesy]

/backport to stable28