Make loadByForeignKey() and related methods return Optional

This is safer and addresses a common source of confusion in the codebase because it's always explicit that the resource returned may not be present, whether because it's soft-deleted when projected to the given time or because it never existed in the first place.

In production code, the presence of the returned value is always checked. In test code, its presence is assumed using .get() where that is expected and convenient, as it not being present will throw an NPE that will cause the test to fail anyway.

Note that the roughly equivalent reloadResourceByForeignKey(), which is widely used in test code, is not having this same treatment applied to it. That is out of the scope of this CL, and has much smaller returns anyway because it's only used in tests (where the unexpected absence of a given resource would just cause the test to fail).

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=225424002

java/google/registry/rde/imports/RdeHostLinkAction.java

@@ -195,11 +195,14 @@ public class RdeHostLinkAction implements Runnable {
               .stream()
               .skip(hostName.parts().size() - (tld.get().parts().size() + 1))
               .collect(joining("."));
-      DomainResource superordinateDomain = loadByForeignKey(DomainResource.class, domainName, now);
+      Optional<DomainResource> superordinateDomain =
+          loadByForeignKey(DomainResource.class, domainName, now);
       // Hosts can't be linked if domains import hasn't been run
       checkState(
-          superordinateDomain != null, "Superordinate domain does not exist: %s", domainName);
-      return Optional.of(superordinateDomain);
+          superordinateDomain.isPresent(),
+          "Superordinate domain does not exist or is deleted: %s",
+          domainName);
+      return superordinateDomain;
     }
   }
 
@@ -209,10 +212,4 @@ public class RdeHostLinkAction implements Runnable {
     SUPERORDINATE_DOMAIN_IN_PENDING_DELETE,
     HOST_LINKED;
   }
-
-  private static class HostLinkException extends RuntimeException {
-    HostLinkException(String hostname, String xml, Throwable cause) {
-      super(String.format("Error linking host %s; xml=%s", hostname, xml), cause);
-    }
-  }
 }