[debug:breakpoints] Make breakpoint.manager optional. (#3450)

Author: jmolivas

config/services/debug.yml

@@ -92,7 +92,7 @@ services:
       - { name: drupal.command }
   console.breakpoints_debug:
     class: Drupal\Console\Command\Debug\BreakpointsCommand
-    arguments: ['@breakpoint.manager', '@app.root']
+    arguments: ['@?breakpoint.manager', '@app.root']
     tags:
       - { name: drupal.command }
   console.cache_context_debug: