[PDI-10983] Fix SwitchCaseMeta load/save

Forward port of [SP-837]
Use deserialized target stepname as default if target step has not yet been resolved.
Use empty string as default target case value.
Fixes applied to XML and Repo save/load.

http://jira.pentaho.com/browse/PDI-10983
http://jira.pentaho.com/browse/SP-837
[PDI-10983] Switch/Case StepMeta Load/Save unit test

Forward port of [SP-837]
Load and save StepMeta from repo and xml. Verify attribute preservation.

http://jira.pentaho.com/browse/SP-837
http://jira.pentaho.com/browse/PDI-10983
Merge pull request #259 from hudak/pdi-10983

[PDI-10983] Fix SwitchCaseMeta load/save