Mastodon sharing button does not properly handle cancellation and empty input

[Marcono1234]

Description

The recently introduced button for sharing a video on Mastodon does not properly handle when the browser dialog for the Mastodon domain is cancelled or the input field is left empty.

In both cases it constructs incorrect URLs and tries to open them.

Expected behavior

When the browser dialog is cancelled or an empty value is provided by the user, sharing should be aborted and no website should be opened.

Reproduction steps

1. Open for example https://media.ccc.de/v/38c3-wir-wissen-wo-dein-auto-steht-volksdaten-von-volkswagen
2. Click the Mastodon sharing button at the bottom of the screen
   ℹ️ Note: A browser dialog appears, asking you for the domain of the Mastodon server
3. Click "Cancel" on the dialog
   ❌ Bug: It uses null as domain

[QbDesu]

Was fixed in 535efbf