Cómo agrupar condicionalmente en una columna sin usar FULL OUTER JOIN

Esta es una agregación condicional simple por lo que puedo decir:

select id, 
       array_agg(amount) filter (where type = 'Customer') as customer_array,
       sum(amount) filter (where type = 'Customer') as customer_sum,
       array_agg(amount) filter (where type = 'Partner') as partner_array,
       sum(amount) filter (where type = 'Partner') as partner_sum
from table_a
group by id;

Si desea una matriz vacía en lugar de un NULL valor, envuelva las funciones de agregación en un coalesce() :

select id, 
       coalesce((array_agg(amount) filter (where type = 'Customer')),'{}') as customer_array,
       coalesce((sum(amount) filter (where type = 'Customer')),0) as customer_sum,
       coalesce((array_agg(amount) filter (where type = 'Partner')),'{}') as partner_array,
       coalesce((sum(amount) filter (where type = 'Partner')),0) as partner_sum
from table_a
group by id;