Add the ability to nullify the entire billing account map (#1630)

core/src/main/java/google/registry/tools/CreateOrUpdateRegistrarCommand.java

@@ -173,7 +173,8 @@ abstract class CreateOrUpdateRegistrarCommand extends MutatingCommand {
           "Registrar Billing Account key-value pairs (formatted as key=value[,key=value...]), "
               + "where key is a currency unit (USD, JPY, etc) and value is the registrar's billing "
               + "account id for that currency. During update, only the pairs that need updating "
-              + "need to be provided.",
+              + "need to be provided, except when an empty string is provided, in which case the"
+              + "entire map is nullified.",
       converter = CurrencyUnitToStringMap.class,
       validateWith = CurrencyUnitToStringMap.class)
   private Map<CurrencyUnit, String> billingAccountMap;
@@ -368,7 +369,9 @@ abstract class CreateOrUpdateRegistrarCommand extends MutatingCommand {
       Optional.ofNullable(poNumber).ifPresent(builder::setPoNumber);
       if (billingAccountMap != null) {
         LinkedHashMap<CurrencyUnit, String> newBillingAccountMap = new LinkedHashMap<>();
-        if (oldRegistrar != null && oldRegistrar.getBillingAccountMap() != null) {
+        if (oldRegistrar != null
+            && oldRegistrar.getBillingAccountMap() != null
+            && !billingAccountMap.isEmpty()) {
           newBillingAccountMap.putAll(oldRegistrar.getBillingAccountMap());
         }
         newBillingAccountMap.putAll(billingAccountMap);