bug in filtering

[emilefokkema]

Reproduction:

• use the following list of options on the 'source' side:

[
    {id: 'a'},
    {id: 'b'},
    {id: null},
    {id: 'c'}
]

• turn filtering on
• try to filter the source on 'a'

Expected: only the 'a' options remains on the 'source' side
Actual: all options remain visible on the 'source' side

See this commit for a live reproduction