Merging two table in sqlplus

I have two tables with same structure:
masterTable (id, description, list_price)
and
changesTable (id, description, list_price)

I want to merge changes from changesTable into masterTable, but I don't want to update 'description' in masterTable when it is NULL in changesTable, I want to keep old value

I tried to do this

WHEN MATCHED THEN
 UPDATE SET master.list_price=changes.list_price,
 master.description=ch.description   
 WHERE length(changes.description)>0
 

but in this case the list_price doesn't get updated as well.

How can I merge them properly?
Thanks

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