Fix referenced before assignment in role requirements

lib/ansible/playbook/role/requirement.py

@@ -190,8 +190,8 @@ class RoleRequirement(RoleDefinition):
 
         def run_scm_cmd(cmd, tempdir):
             try:
-                stdout = None
-                stderr = None
+                stdout = ''
+                stderr = ''
                 popen = Popen(cmd, cwd=tempdir, stdout=PIPE, stderr=PIPE)
                 stdout, stderr = popen.communicate()
             except Exception as e: