handle case where stdout is the empty string

procs.py

@@ -30,9 +30,8 @@ class Process(object):
 
     @property
     def stdout(self):
-        if self._stdout:
+        if self._stdout is not None:
             return self._stdout
-        return 'stdout'
 
     @property
     def stderr(self):

tests/test_procs.py

@@ -15,6 +15,12 @@ def test_single_proc():
     assert ls.stdout == u'file1\nfile2\nfile3\n'
 
 
+def test_empty_output():
+    cat = Process('cat /dev/null')
+    cat.run()
+    assert cat.stdout == u''
+
+
 def test_returncode():
     assert not os.path.exists('/bin/nosuchcommand')