Replace negated 'isEmpty' with 'isNotEmpty'

src/commonMain/kotlin/client/Session.kt

@@ -84,7 +84,7 @@ class Session {
     fun openCollection(collectionUId: CollectionUId, readOnly: Boolean): Collection {
         if (ActiveTransaction != null) throw RuntimeException("A collection cannot be open within a transaction.")
         if (this.isClosed) throw RuntimeException("This session has been closed.")
-        if (!this.openedCollections.isEmpty()) throw RuntimeException("A collection is alredy opened.")
+        if (this.openedCollections.isNotEmpty()) throw RuntimeException("A collection is alredy opened.")
 
         val newCollection = Collection(this, collectionUId, readOnly)
         this.openedCollections.put(collectionUId, newCollection)