[Phone book] Error when filtering by condition 'contains empty value'

[pmallol]

Found in PROD Version 3.0.3 and STG 3.1.1-3-gd29a265

Preconditions:
There is an active project with phone numbers listed in the Phone Book

1. Open Project
2. Navigate to 'Phone Book' tab
3. In Filters, add a condition "'Language' contains value [empty]" and click the checkmark to actually add the condition.
4. Click the 'Search' button to filter the listing
5. Change the condition to "'Phone number' contains value [empty]" and click the checkmark to actually add the condition.

Obtained:
A 500 error appears in the console for both conditions.
In step 3., Language fields are empty and could appear as possible results to condition "'Language' contains value [empty]" if [empty] was a valid value.