It looks like you're hitting a defect #14725210 (PL/SQL ONLY :STRINGS ARE RETURNED WITH UNEXPECTED NULL VALUES FROM DG4MSQL) which is currently being worked on where a similar issue has already been reported.
There's no fix yet available. You could check out the issue on My Oracle Support and track the progress on this issue or you can also file a service request to get a confirmation that you exactly hit this issue and get alerted as soon as the issue is fixed.
Thanks, I have looked defect report 14725212 and opened an SR with Oracle Support, however, I am also seeing this on a Windows 2008 R2 server using the dg4ODBC driver version 126.96.36.199. Would you know if this bug affects the dbgODBC driver too?
For DG4ODBC there's another similar issue reported: BUG 13950747 - NCHAR/NVARCHAR USING DG4MSQL ADDS A SPACE/BLANK TO EVERY CHARACTER USING PL/SQL
It was mentioned somewhere in the bug that it happens also with varchar in a Unicode env.