SP-1117 - Backport of PDI-11420 - Cannot reopen Mapping (sub-transformation) step after saving with blank value for fieldname to target step. (5.0)

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.

junit test added