Avoided null error on count

This commit avoids a warning when the library tries to call count() on a
non-array object. The issue is avoided by checking whether the object is
an array beforehand.

See #51

src/Command/Delivery/DebugCommand.php

@@ -52,7 +52,7 @@ public function __construct($clients, $configurations = [])
         $this->setDescription('Shows information about data coming from a certain client');
         $this->addArgument(
             'client-name',
-            \count($this->clients) > 1 ? InputArgument::REQUIRED : InputArgument::OPTIONAL,
+            \is_array($this->clients) && \count($this->clients) > 1 ? InputArgument::REQUIRED : InputArgument::OPTIONAL,
             'The name of the client to use'
         );
     }