Power cell slot QOL (#15373)

Content.Server/PowerCell/PowerCellSlotEmptyEvent.cs

@@ -0,0 +1,7 @@
+namespace Content.Server.PowerCell;
+
+/// <summary>
+/// Raised directed on an entity when its active power cell has no more charge to supply.
+/// </summary>
+[ByRefEvent]
+public readonly record struct PowerCellSlotEmptyEvent;