Fix dead players sending alliance requests (#1508)

## Description:

Prevent unholly alliances with the undead.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I have read and accepted the CLA aggreement (only required once).

## Please put your Discord username so you can be contacted if a bug or
regression is found:

IngloriousTom

src/core/game/PlayerImpl.ts

@@ -398,7 +398,7 @@ export class PlayerImpl implements Player {
     if (other === this) {
       return false;
     }
-    if (this.isFriendly(other)) {
+    if (this.isFriendly(other) || !this.isAlive()) {
       return false;
     }