Changed check command exists to only return a boolean

Author: bstoney

src/System/UnixSystem.php

@@ -47,7 +47,7 @@ public function checkIfCommandExists($cmdName)
             "if command -v ".$cmdName." >/dev/null 2>&1; then echo \"true\"; else echo \"false\"; fi;"
         );
 
-        return filter_var($sysResponse, FILTER_VALIDATE_BOOLEAN, FILTER_NULL_ON_FAILURE);
+        return filter_var($sysResponse, FILTER_VALIDATE_BOOLEAN);
     }
 
     /** @return string|null */

src/System/WindowsSystem.php

@@ -45,7 +45,7 @@ public function checkIfCommandExists($cmdName)
 
         $sysResponse = exec("where $cmdName > nul 2>&1 && echo true");
 
-        return filter_var($sysResponse, FILTER_VALIDATE_BOOLEAN, FILTER_NULL_ON_FAILURE);
+        return filter_var($sysResponse, FILTER_VALIDATE_BOOLEAN);
     }
 
     /** @return string */