Keep Playing players valid when xesam:title is transiently empty

[RobRobM]

Summary

Chromium-based browsers (Brave, Chrome, etc.) that forward MediaSession to MPRIS sometimes publish an empty Metadata dict while PlaybackStatus stays Playing — typically during track transitions on streaming sites such as Radio Paradise. The empty window can last several seconds before real metadata reappears.

The current validatePlayer() marks a player invalid whenever xesam:title is missing. That causes the panel to drop the actually-playing browser player and switch to another (often paused) source, and it doesn't recover cleanly when the title returns.

Changes

src/helpers/shell/PlayerProxy.js

• Relax validatePlayer(): a player is only invalid when it has no title and is not currently Playing. A Playing player with empty metadata is still a legitimate candidate.
• Re-run validatePlayer() on PlaybackStatus changes, so a Paused-without-title player that later starts playing becomes valid again (and vice versa).

src/helpers/shell/PanelButton.js

• Default xesam:title to "" at the two sites that pass it into ScrollingLabel / labelTextElements. Without this, GObject throws Invalid value 'undefined' for property text when an empty-metadata Playing player reaches the panel widgets, and updateWidgets aborts — which defeats the point of keeping the player valid.

Root cause / verification

Verified directly on the bus with gdbus call --session --dest org.mpris.MediaPlayer2.brave.instance_X --object-path /org/mpris/MediaPlayer2 --method org.freedesktop.DBus.Properties.Get org.mpris.MediaPlayer2.Player Metadata during a Radio Paradise track transition — the returned a{sv} is {} for up to several seconds while PlaybackStatus is Playing. This is not a stale-cache issue inside the extension; the publisher's DBus state is genuinely empty.

Test plan

• Start Radio Paradise in Brave (GNOME 49, Wayland). Panel shows Brave as active.
• Skip to next track. Panel keeps Brave selected through the empty-metadata gap instead of switching away.
• journalctl --user -f | grep gnome-shell stays clean (no ScrollingLabel GObject errors, no playerProxy is null).
• When a second player (Firefox/YouTube) starts/stops, selection logic still prefers the playing one.

Notes

• Independent of Fix race: guard this.playerProxy against becoming null during awaits in PanelButton #303 (PanelButton race fix) — branch is based on main, no merge conflicts expected either way.
• A follow-up PR is planned to cache last-known-good metadata per player so the panel can keep showing the previous title/artist during these empty windows instead of just a blank label. This PR only fixes the "player gets dropped" side of the bug.