custom list prefix and exclusion ignore "empty value"

[Katja]

Composite custom list can get prefix or be excluded if any or undefined. Later is same as "no value" or "empty".

But that is not possible for custom list i.e. list with values instead of being omposed.
The following will allow to select/keep first entry of a list as this will now be treated as "no/empty value.

The following two rows will solve this for prefix and exclusion

in \src\calibre\gui2\catalog\catalog_epub_mobi.py
at 141 add two rows
if rule['pattern'] == '':
    rule['pattern'] = 'None'

# showing existing follower rows for orientation only
if not rule['enabled']:

Hi @kovidgoyal,
do you mind to implement this?

[Kovid Goyal (kovid)]

That would change the meaning of empty rule from "rule is ignored" to
"exclude empty values". That would change the way the GUI works. This is
particularly bad since there are default rules with empty values.

[Katja (katjawy)]

agree.

Would be nice to keep this open.
I will try to find a better solution as I still like to use prefix/exclusion when value is emtpty.