docs: handle empty flags sections on the feature flags page (#20222)

Author: sethamus

docs/src/pages/flags.md

@@ -34,6 +34,7 @@ The following policies apply to `unstable_` flags.
 
 ## Active Flags
 
+{% if flags.active | length %}
 The following flags are currently available for use in ESLint.
 
 <table>
@@ -49,9 +50,13 @@ The following flags are currently available for use in ESLint.
 {%- endfor -%}
  </tbody>
 </table>
+{% else %}
+There are currently no active flags.
+{% endif %}
 
 ## Inactive Flags
 
+{% if flags.inactive | length %}
 The following flags were once used but are no longer active.
 
 <table>
@@ -68,6 +73,9 @@ The following flags were once used but are no longer active.
 {%- endfor -%}
  </tbody>
 </table>
+{% else %}
+There are currently no inactive flags.
+{% endif %}
 
 ## How to Use Feature Flags