[web] Disabled edit button if edited message is empty

Summary: We want to match editing behavior with the native. If the user types an empty message, the edit button will be disabled, and the user will be prevented from saving the changes.

Test Plan:

• Entered edit mode of a message.
• Cleared the whole message.
• Checked if the edit button is disabled.
• Typed some spaces.
• Checked if the edit button is still disabled.
• Typed some text.
• Checked if the edit button is enabled.

Reviewers: michal, inka

Reviewed By: michal

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7971