refactor: Optimize arraylist instanciation. (#1502)

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Fixes #<issue_number_goes_here> ☕️

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Author: NomadeLampe

google-cloud-firestore/src/main/java/com/google/cloud/firestore/QuerySnapshot.java

@@ -206,9 +206,9 @@ public Iterator<QueryDocumentSnapshot> iterator() {
    */
   @Nonnull
   public <T> List<T> toObjects(@Nonnull Class<T> clazz) {
-    List<T> results = new ArrayList<>();
-
-    for (DocumentSnapshot documentSnapshot : getDocuments()) {
+    List<QueryDocumentSnapshot> documents = getDocuments();
+    List<T> results = new ArrayList<>(documents.size());
+    for (DocumentSnapshot documentSnapshot : documents) {
       results.add(
           CustomClassMapper.convertToCustomClass(
               documentSnapshot.getData(), clazz, documentSnapshot.getReference()));