Prevent custom columns from overriding the information shown in the book details panel

src/calibre/gui2/library/models.py

@@ -358,8 +358,9 @@ class BooksModel(QAbstractTableModel): # {{{
             name, val = mi.format_field(key)
             if mi.metadata_for_field(key)['datatype'] == 'comments':
                 name += ':html'
-            if val:
+            if val and name not in data:
                 data[name] = val
+
         return data