[...] WHERE
    (
        attribute_1 LIKE '%foo%'
        AND
        attribute_2 LIKE '%bar%'
    )
    OR
    (
        attribute_1 LIKE '%foo%'
        AND
        attribute_2 is NULL
    )

$categories = Mage::getModel('catalog/category')
    ->getCollection()
    ->addAttributeToSelect('*')
    ->addAttributeToFilter(
        array(
            array(
                array(
                    'attribute' => 'attribute_1',
                    'like' => '%foo%'
                ),
                array(
                    'attribute' => 'attribute_2',
                    'like' => '%bar%'
                ),
            ),
            array(
                array(
                    'attribute' => 'attribute_1',
                    'like' => '%foo%'
                ),
                array(
                    'attribute' => 'attribute_2',
                    'null' => true
                ),
            ),
        )
    );

(
    attribute_1 LIKE '%foo%'
    AND
    attribute_2 LIKE '%bar%'
)
OR
(
    attribute_1 LIKE '%foo%'
    AND
    attribute_2 is NULL
)

attribute_1 LIKE '%foo%'
AND
(
    attribute_2 LIKE '%bar%'
    OR
    attribute_2 is NULL
)

$collection->addAttributeToFilter('attribute_1', ['like' => '%foo%']);
$collection->addAttributeToFilter('attribute_2', [['like' => '%bar%'], ['null' => true]]);

$collection->getSelect()->where(
    "(attribute_1 LIKE '%foo%' AND attribute_2 LIKE '%bar%') OR
    attribute_1 LIKE '%baz%' AND attribute_2 is NULL"
);

WHERE (...)
AND
(
    (
        attribute_1 LIKE '%foo%'
        AND
        attribute_2 LIKE '%bar%'
    ) OR (
        attribute_1 LIKE '%baz%'
        AND
        attribute_2 is NULL
    )
)