Fix nation spawnkilling 🔧 (#3222)

## Description:

As far as I can remember, in v28 the spawn immunity applied to both
humans and nations.
With the configurable spawn immunity (added for v29) the spawn immunity
no longer applies to nations... Because its called PVP immunity now.
So right now it's possible to spawnkill nations. This is a big problem
for the 5M gold modifier games... And you can "cheat" in singleplayer.

This PR changes two things:
- Nations always have 5 seconds spawn immunity now, no matter whats
configured for the PVP immunity
- Nations attack TerraNullius earlier (Otherwise the easy nations would
sometimes do their first attack after the 5 seconds are over, spawnkills
would still be possible)

## 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

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

FloPinguin

---------

Co-authored-by: Ryan <7389646+ryanbarlow97@users.noreply.github.com>

tests/Attack.test.ts

@@ -426,11 +426,29 @@ describe("Attack immunity", () => {
     expect(playerA.units(UnitType.TransportShip)).toHaveLength(1);
   });
 
-  test("Should be able to attack nations during immunity phase", async () => {
+  test("Should not be able to attack nations during nation immunity phase", async () => {
+    (game.config() as TestConfig).setNationSpawnImmunityDuration(
+      immunityPhaseTicks,
+    );
     const nationId = "nation_id";
     const nation = new PlayerInfo("nation", PlayerType.Nation, null, nationId);
     game.addPlayer(nation);
-    // Player A attacks the nation
+    // Player A attacks the nation during nation immunity
+    const attackExecution = new AttackExecution(null, playerA, nationId, null);
+    game.addExecution(attackExecution);
+    game.executeNextTick();
+    expect(playerA.outgoingAttacks()).toHaveLength(0);
+  });
+
+  test("Should be able to attack nations after nation immunity phase", async () => {
+    (game.config() as TestConfig).setNationSpawnImmunityDuration(
+      immunityPhaseTicks,
+    );
+    const nationId = "nation_id";
+    const nation = new PlayerInfo("nation", PlayerType.Nation, null, nationId);
+    game.addPlayer(nation);
+    waitForImmunityToEnd();
+    // Player A attacks the nation after immunity
     const attackExecution = new AttackExecution(null, playerA, nationId, null);
     game.addExecution(attackExecution);
     game.executeNextTick();