Fix inability to use zero delay (#44)

The default config has start_delay as zero, which is skipped over by the if condition due to zero's falseness

Author: KaelenProctor

src/PhiremockExtension/Config.php

@@ -208,7 +208,7 @@ private function initDelay(array $config): void
             return;
         }
 
-        if ($config['start_delay']) {
+        if (is_int($config['start_delay']) && $config['start_delay'] >= 0) {
             $this->delay = (int) $config['start_delay'];
         }
     }