Fix NPE in finishGame

Apparently, transaction handlers are very often called with null even if
the object exists as an optimization. See:

https://groups.google.com/g/firebase-talk/c/Lyb0KlPdQEo
https://stackoverflow.com/a/65415636/5190601

Fixes: #4

functions/src/index.ts

@@ -49,13 +49,6 @@ export const finishGame = functions.https.onCall(async (data, context) => {
     .ref(`gameData/${gameId}`)
     .once("value");
   const gameSnap = await admin.database().ref(`games/${gameId}`).once("value");
-  if (!gameSnap.exists()) {
-    throw new functions.https.HttpsError(
-      "not-found",
-      `The game with gameId ${gameId} was not found in the database.`
-    );
-  }
-
   const gameMode = (gameSnap.child("mode").val() as GameMode) || "normal";
 
   const { lastSet, deck, finalTime, scores } = replayEvents(gameData, gameMode);
@@ -73,6 +66,14 @@ export const finishGame = functions.https.onCall(async (data, context) => {
     .database()
     .ref(`games/${gameId}`)
     .transaction((game) => {
+      // Transaction handler should always handle null, because firebase often
+      // calls it like that: https://stackoverflow.com/a/65415636/5190601
+      if (game === null) {
+        throw new functions.https.HttpsError(
+          "not-found",
+          `The game with gameId ${gameId} was not found in the database.`
+        );
+      }
       if (game.status !== "ingame") {
         // Someone beat us to the atomic update, so we cancel the transaction.
         return;