You can use a combination of transforms to form a pipeline of operations that will get you what you want. The order of these operations should be:
right pad transform → substring transform → trim transform
The right pad transform will add white-space to the end of DEPT_NAME. This will prevent your substring transform from returning null, since all strings will now have at least 64 characters. Finally, you use the trim operation to trim off the whitespace from your padded DEPT_NAMES.