The log-text parameter is required according to the documentation,
but it's perfectly valid for it to be an empty string or null.
If the value is null in the yaml, the Python data structure
contains None. The code was converting this directly to a string,
resulting in 'None' in the xml output.
This change makes sure that an empty string is used instead,
resulting in a correct empty element in the xml.
Change-Id: Ided872f5528bae93347f96bd729b5ce37f8e16fb
Nicola Heald