fix: don't throw NPE in DocumentSnapshot.getDate() when value is missing (#513)

Author: schmidt-sebastian

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

@@ -345,7 +345,8 @@ public Long getLong(@Nonnull String field) {
  */
  @Nullable
  public Date getDate(@Nonnull String field) {
- return ((Timestamp) get(field)).toDate();
+ Timestamp timestamp = getTimestamp(field);
+ return timestamp == null ? null : timestamp.toDate();
  }
 
  /**