Remove @EmptySetToNull (#1878)

This annotation was only used in cross-database comparison.

<!-- Reviewable:start -->
- - -
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/google/nomulus/1878)
<!-- Reviewable:end -->

core/src/main/java/google/registry/model/ImmutableObject.java

@@ -51,15 +51,6 @@ public abstract class ImmutableObject implements Cloneable {
   @Target(FIELD)
   public @interface DoNotHydrate {}
 
-  /**
-   * Indicates that the field stores a null value to indicate an empty set. This is also used in
-   * object comparison.
-   */
-  @Documented
-  @Retention(RUNTIME)
-  @Target(FIELD)
-  public @interface EmptySetToNull {}
-
   /**
    * Indicates that the field does not take part in the immutability contract.
    *

core/src/main/java/google/registry/model/domain/DomainBase.java

@@ -124,7 +124,7 @@ public class DomainBase extends EppResource
   String tld;
 
   /** References to hosts that are the nameservers for the domain. */
-  @EmptySetToNull @Transient Set<VKey<Host>> nsHosts;
+  @Transient Set<VKey<Host>> nsHosts;
 
   /** Contacts. */
   VKey<Contact> adminContact;

core/src/main/java/google/registry/model/domain/DomainHistory.java

@@ -96,7 +96,6 @@ public class DomainHistory extends HistoryEntry {
               columnList =
                   "domain_history_history_revision_id,domain_history_domain_repo_id,host_repo_id",
               unique = true))
-  @EmptySetToNull
   @Column(name = "host_repo_id")
   Set<VKey<Host>> nsHosts;
 
@@ -160,7 +159,6 @@ public class DomainHistory extends HistoryEntry {
   @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, orphanRemoval = true)
   @JoinColumn(name = "historyRevisionId", referencedColumnName = "historyRevisionId")
   @JoinColumn(name = "domainRepoId", referencedColumnName = "domainRepoId")
-  @EmptySetToNull
   Set<DomainTransactionRecord> domainTransactionRecords;
 
   public Set<DomainTransactionRecord> getDomainTransactionRecords() {