fix(plugins): return null when room is not selected in plugin instead of throwing an exception

[ShGKme]

☑️ Resolves

• Regression: refactor(plugins): add reusable requestRoomSelection util to run room selector #12356
• Fix: Actually do what util's documentation says and plugins expect
• Doesn't seem to really break anything, but annoying error in the console

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After

🏁 Checklist

• 🌏 Tested with Chrome, Firefox and Safari or should not be risky to browser differences
• 🖥️ Tested with Desktop client or should not be risky for it
• 🖌️ 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