OpenXmlFilter: Conditionally allow empty targets, do not copy source text into targets

[dionissds]

We need the OpenXmlFilter to have the behavior to allow empty targets. As it is now, the source text is always copied into targets, in case they are null.

It appears that making this change is not very invasive and it consists of making a few changes to the StyledTextSkeletonWriter class and ConditionalParameters.

Apparently, the method getTargetForOutput() inside the class does exactly what we want to avoid (use the source when the target is empty).

Our solution will consist to make this behaviour conditional and add a new flag to the ConditionalParameters class.

Similar behaviour is mentioned in issue #1055 regarding OmegaT with Okapi filter OpenXML.

[dionissds]

Created PR to Okapi repo related to this issue: https://bitbucket.org/okapiframework/okapi/pull-requests/579/1105-added-a-new-paramter-to-allow-empty

[dionissds]

Created a new PR that only contains the AllowEmptyTarget parameter: https://bitbucket.org/okapiframework/okapi/pull-requests/599/1105-added-a-new-paramter-to-allow-empty

The old PR is blocked since the changes for the default Okapi’s empty target behavior caused many integration tests to fail.