Fix minor code style issues

Author: Martin Qvistgård

src/Command.php

@@ -337,15 +337,15 @@ public function execute()
                 1   => array('pipe','w'),
                 2   => array('pipe', $this->getIsWindows() ? 'a' : 'w'),
             );
-            if (isset($this->_stdIn)) {
+            if ($this->_stdIn!==null) {
                 $descriptors[0] = array('pipe', 'r');
             }
 
             $process = proc_open($command, $descriptors, $pipes, $this->procCwd, $this->procEnv, $this->procOptions);
 
             if (is_resource($process)) {
 
-                if(isset($this->_stdIn)) {
+                if($this->_stdIn!==null) {
                     fwrite($pipes[0], $this->_stdIn);
                     fclose($pipes[0]);
                 }