Improve mysql fetch_field* return type

[MoonE]

While a table column name cannot be null, it is still possible to use an empty string as an alias. In this case name is an empty string.

SELECT'';

def and catalog are constants, and max_length is always 0 since php 8.1 https://www.php.net/manual/en/mysqli-result.fetch-field.php#refsect1-mysqli-result.fetch-field-returnvalues