flexible attributes: Is the attribute empty or undefined

When you search for a flexible attribute it is not clear if the value is empty or undefined:

```
dfc@ronin:~/0xRepos/Audio$ beet list sometagthatimadeup:|head
/Passenger - Wicked Man’s Rest - Wicked Man's Rest
/Passenger - Wicked Man’s Rest - Night Vision Binoculars
/Passenger - Wicked Man’s Rest - Things You've Never Done
/Passenger - Wicked Man’s Rest - Girl I Once Knew
/Passenger - Wicked Man’s Rest - Do What You Like
/Passenger - Wicked Man’s Rest - Needle in the Dark
/Passenger - Wicked Man’s Rest - Four Horses
/Passenger - Wicked Man’s Rest - You're on My Mind
/Passenger - Wicked Man’s Rest - For You
/Passenger - Wicked Man’s Rest - Walk in the Rain
dfc@ronin:~/0xRepos/Audio$ beet list sometagthatimadeup:""|head
/Passenger - Wicked Man’s Rest - Wicked Man's Rest
/Passenger - Wicked Man’s Rest - Night Vision Binoculars
/Passenger - Wicked Man’s Rest - Things You've Never Done
/Passenger - Wicked Man’s Rest - Girl I Once Knew
/Passenger - Wicked Man’s Rest - Do What You Like
/Passenger - Wicked Man’s Rest - Needle in the Dark
/Passenger - Wicked Man’s Rest - Four Horses
/Passenger - Wicked Man’s Rest - You're on My Mind
/Passenger - Wicked Man’s Rest - For You
/Passenger - Wicked Man’s Rest - Walk in the Rain
dfc@ronin:~/0xRepos/Audio$

```

ore me personally there is a difference between an empty value and a ield that is not present.

Start by reproducing the two `beet list` queries shown in the issue and inspect how flexible-attribute searches distinguish an omitted field from an empty value. Done means the query behavior clearly returns different results for undefined and explicitly empty attributes, with the expected behavior covered by tests.

Written by the indexing model from the issue text.