Finally, this meta_query works :
'meta_query' => array(
'relation' => 'OR',
array(
'key' => 'votes_average',
'value' => '6',
'compare' => '<',
'type' => 'NUMERIC',
),
array(
'key' => 'votes_average',
'compare' => 'NOT EXISTS',
'value' => 'null',
)
),