oeqa: fix OETestCalledProcessError for check_output method

oeqa: fix OETestCalledProcessError for check_output method

Per documentation, subprocess.CalledProcessError exception has stderr
filled out only for run method, it's None for check_output method.
So serialize it only if it's not None.

Avoids:
  File "<poky-dir>/meta/lib/oeqa/utils/subprocesstweak.py", line 15, in __str__
    s = s + "\nStandard Error: " + strify(self.stderr)
        ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
TypeError: can only concatenate str (not "NoneType") to str

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>