MariaDB server crashes when a query includes a derived table
containing unnamed column (eg: `SELECT '' from t`). When `Item`
object representing such unnamed column was checked for valid,
non-empty name in `TABLE_LIST::create_field_translation`, the
server crahsed(assertion `item->name.str && item->name.str[0]`
failed).
This fix removes the redundant assertion. The assert was a strict
debug guard that's no longer needed because the code safely handles
empty strings without it.
Selecting `''` from a derived table caused `item->name.str`
to be an empty string. While the pointer itself wasn't `NULL`
(`item->name.str` is `true`), its first character (`item->name.str[0]`)
was null terminator, which evaluates to `false` and eventually made
the assert fail. The code immediately after the assert can safely
handle empty strings and the assert was guarding against something
which the code can already handle.
Includes `mysql-test/main/derived.test` to verify the fix.
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