Make sure SnapshotListener on Account is actually added (#46)

# Make sure SnapshotListener on Account is actually added

## ♻️ Current situation & Problem
I noticed that there actually never is a snapshot listener on Account. I found out that it is only added in the function `FirestoreAccountStorage.snapshotListener(for:with:)`, which previously only created a snapshot listener, if there already is one - so never. Changing the operator will solve the problem and makes sure that we add the snapshot listener when needed.


## ⚙️ Release Notes 
- Make sure to add snapshot listener on Account.


## 📚 Documentation
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## ✅ Testing
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Sources/SpeziFirebaseAccountStorage/FirestoreAccountStorage.swift

@@ -196,7 +196,7 @@ public actor FirestoreAccountStorage: AccountStorageProvider {
         let localCache = localCache
         let cached = await localCache.loadEntry(for: accountId, keys)
 
-        if listenerRegistrations[accountId] != nil { // check that there is a snapshot listener in place
+        if listenerRegistrations[accountId] == nil { // check that there is a snapshot listener in place
             snapshotListener(for: accountId, with: keys)
         }