Refactor temperature retrieval with None check

[digidolphin]

Problem

When the target temperature is set to 0 and Home Assistant restarts,
the climate entity resets to 21 (DEFAULT_TEMP). Any other value
(e.g. 1–15) is restored correctly — only 0 is affected.

Cause

In async_added_to_hass the restored temperature is checked for
truthiness via the walrus operator:

if temperature := previous_state.attributes.get(ATTR_TEMPERATURE, DEFAULT_TEMP):
    self._attr_target_temperature = float(temperature)

When the stored value is 0, the condition evaluates to False
(0 is falsy in Python), so the restore is skipped and the entity
keeps the DEFAULT_TEMP (21) assigned in __init__.

Fix

Check explicitly for None instead of truthiness:

if (temperature := previous_state.attributes.get(ATTR_TEMPERATURE)) is not None:
    self._attr_target_temperature = float(temperature)

The DEFAULT_TEMP fallback in .get() is redundant and has been
removed: if the attribute is missing, the default set in __init__
already applies.

Testing

• Set target temperature to 0, restart HA → 0 is now restored
• Set target temperature to e.g. 5 or 21, restart HA → still restored correctly
• No previous state (fresh entity) → defaults to 21 as before