virttest.utils_env: Check the whether the key is none before using it.

When get vm objects from Env object, the key of the data maybe None.
if the key is None, will raise error like:
   'NoneType' object has no attribute 'startswith'.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
Merge pull request #1109 from humanux/env_issue

virttest.utils_env: Check the whether the key is none before using it.