Fix collections not displaying in device list for KoboTouch driver

Collections were not being shown in the device list if either the
collection management was off in the driver, or no collections column
were configured. There were two reasons, one was the empty configuration
string was handled incorrectly. The other was the device_collections
attribute could be cleared by the smart_update.

src/calibre/devices/kobo/driver.py

@@ -3052,7 +3052,12 @@ class KOBOTOUCH(KOBO):
 
     @property
     def collections_columns(self):
-        return self.get_pref('collections_columns')
+        return self.get_pref('collections_columns') if self.manage_collections else ''
+
+    def get_collections_attributes(self):
+        collections_str = self.collections_columns
+        collections = [x.lower().strip() for x in collections_str.split(',')] if collections_str else []
+        return collections
 
     @property
     def delete_empty_collections(self):

src/calibre/ebooks/metadata/book/__init__.py

@@ -113,7 +113,7 @@ SC_FIELDS_NOT_COPIED =     frozenset(['title', 'title_sort', 'authors',
                                       'identifiers'])
 
 # Metadata fields that smart update should copy only if the source is not None
-SC_FIELDS_COPY_NOT_NULL =  frozenset(['lpath', 'size', 'comments', 'thumbnail'])
+SC_FIELDS_COPY_NOT_NULL =  frozenset(['device_collections', 'lpath', 'size', 'comments', 'thumbnail'])
 
 # Metadata fields that smart update should copy without special handling
 SC_COPYABLE_FIELDS =       SOCIAL_METADATA_FIELDS.union(