handle null pkeys in explorer ui

Null primary keys are not supported, however when a null primary key is
present, browsing a selected type in the Hollow Explorer UI will fail
with an OOB exception.

Handle null primary keys so that they are still displayed in the
selected type page, with "NULL" substituted in-place of missing fields.

Author: kvoli

hollow-explorer-ui/src/main/java/com/netflix/hollow/explorer/ui/pages/BrowseSelectedTypePage.java

@@ -167,6 +167,15 @@ private TypeKey getKey(int recordIdx, HollowTypeReadState typeState, int ordinal
                     delimiterEscapedKeyBuilder.append(MULTI_FIELD_KEY_DELIMITER);
                 }
 
+                if (curOrdinal == ORDINAL_NONE) {
+                    // Handle the case where the field is null, display "NULL"
+                    // as the key so the user can take action on it, while still
+                    // rendering the key's non-null fields.
+                    keyBuilder.append("NULL");
+                    delimiterEscapedKeyBuilder.append("NULL");
+                    continue;
+                }
+
                 Object fieldValueObject = HollowReadFieldUtils.fieldValueObject(curState, curOrdinal, fieldPathIndexes[i][fieldPathIndexes[i].length - 1]);
                 keyBuilder.append(fieldValueObject);
                 if (fieldValueObject instanceof String) {