Try the following:
sensor:
- platform: template
sensors:
uptime_symbian:
friendly_name: "Uptime Symbian"
value_template: >-
{% set sensor = states('sensor.tiempo_encendido_trastero') %}
{% set weeks = sensor.split(" week")[0][3:] | int %}
{% set days = sensor.split(" day")[0][-1:] | int + (weeks * 7) %}
{% set hours = sensor.split(" hour")[0][-2:] | int %}
{% set minutes = sensor.split(" minute")[0][-2:] | int %}
{% if days > 0 %}
{{ days }}d:{{ hours }}h:{{ minutes }}m
{% elif hours > 0 %}
{{ hours }}h:{{ minutes }}m
{% else %}
{{ minutes }}m
{% endif %}
This should work in both cases, if it shows 0 or nothing in your sensor.