Logic error in SqlContentEntityStorage::countFieldData() attempts to drop `name` column

Problem/Motivation

The users_field_data table contains a row with uid set to 0. The logic in SqlContentEntityStorage::countFieldData() does a select query looking for a single, distinct uid. Since the first uid in the table is 0, the method always returns false, indicating there is no data in the table.

The query it runs (for a detected change in the name field, for instance):

MariaDB [d8]> SELECT DISTINCT t.uid AS uid FROM users_field_data t WHERE ((name IS NOT NULL)) LIMIT 1 OFFSET 0;
+-----+
| uid |
+-----+
|   0 |
+-----+
1 row in set (0.00 sec)

It then returns as follows:

    return $as_bool ? (bool) $count : (int) $count;

Since the SqlContentEntityStorageSchema::updateSharedTableSchema() method will simply drop and recreate a field without data:

       // Since there is no data we may be switching from a dedicated table
        // to a schema table schema, hence we should use the proper API.
        $this->performFieldSchemaOperation('delete', $original);
        $this->performFieldSchemaOperation('create', $storage_definition);

The resulting error when the 'name' column is attempted to be dropped:

Duplicate entry 'en' for key 'user__name'

Proposed resolution

Do a proper count query, or add special handling for uid=0.

Remaining tasks

User interface changes

API changes

Data model changes