Important: #105441 - TCA select fields with null item values create nullable columns 

Description 

When configuring a TCA select field with renderType => 'selectSingle' and adding an item with 'value' => null, the generated database column is now nullable regardless of whether the other item values are integers or strings.

Previously, the following configuration with integer item values would incorrectly generate a VARCHAR(255) column:

'config' => [
    'type' => 'select',
    'renderType' => 'selectSingle',
    'foreign_table' => 'some_table',
    'default' => null,
    'items' => [
        ['label' => 'Please choose', 'value' => null],
    ],
],

This now correctly generates INT UNSIGNED DEFAULT NULL.

Similarly, a configuration with string item values now also generates a nullable column:

'config' => [
    'type' => 'select',
    'renderType' => 'selectSingle',
    'default' => null,
    'items' => [
        ['label' => 'Default', 'value' => null],
        ['label' => 'Option', 'value' => 'some_value'],
    ],
],

This now correctly generates VARCHAR(255) DEFAULT NULL instead of VARCHAR(255) DEFAULT '' NOT NULL.