Preserve term description when updating (when Associate node body with term description is disabled)

What steps will reproduce the problem?
1. In NAT configuration, disable "Associate node body with term description" checkbox for a node type.
2. Edit a node of that type that has an associated term with a description defined.
3. The term description is now blank.

What is the expected output?
I would like the term description to be preserved, not blanked.

Please provide any additional information below.
I'm attaching a patch with my suggested fix for this issue. I tested my patch and it works for me.