Fix WebUI crash when listing server blocks and rationale is not available (#20408)

Regression from #20391

Fixes #20405

app/javascript/mastodon/features/about/index.js

@@ -191,7 +191,7 @@ class About extends React.PureComponent {
                         <span className='about__domain-blocks__domain__type' title={intl.formatMessage(severityMessages[block.get('severity')].explanation)}>{intl.formatMessage(severityMessages[block.get('severity')].title)}</span>
                       </div>
 
-                      <p>{block.get('comment').length > 0 ? block.get('comment') : <FormattedMessage id='about.domain_blocks.no_reason_available' defaultMessage='Reason not available' />}</p>
+                      <p>{(block.get('comment') || '').length > 0 ? block.get('comment') : <FormattedMessage id='about.domain_blocks.no_reason_available' defaultMessage='Reason not available' />}</p>
                     </div>
                   ))}
                 </div>