PDI-11420 - Cannot reopen Mapping (sub-transformation) step after saving
with blank value for fieldname to target step.

junit test added
PDI-11420 - Cannot reopen Mapping (sub-transformation) step after saving
with blank value for fieldname to target step.

fix added. 
Avoid to set null for rename target/source values.
If somehow this value already set to null - show empty String on UI to
avoid NPE.
Merge pull request #326 from prakapenka/PDI-11420

PDI-11420 - Cannot reopen Mapping (sub-transformation) step after saving with blank value for fieldname to target step.