fix: guard against None in state template handlers

[megakid]

Closes #117

Problem

state_attr() returns None — not a string — when the source entity is missing or unavailable. Every state-handler callback guards only against the strings STATE_UNKNOWN / STATE_UNAVAILABLE, so None falls straight through to float()/int():

File "custom_components/climate_template/climate.py", line 591, in _update_current_temp
    self._attr_current_temperature = float(temp)
TypeError: float() argument must be a string or a real number, not 'NoneType'

Config that triggers it:

- platform: climate_template
  name: "Master Bedroom Auto"
  current_temperature_template: "{{ state_attr('climate.master_bedroom', 'current_temperature') }}"

When the source zone drops out, state_attr yields None. Worth noting this fires on every state change of the source entity while it is unavailable, not only at startup as #117 describes.

Fix

Add None to the sentinel tuple in all 15 handlers.

In the numeric handlers (_update_min_temp, _update_max_temp, _update_current_temp, the four humidity handlers, and the three target-temperature handlers) this prevents the TypeError. In the mode handlers (_update_hvac_mode, _update_preset_mode, _update_fan_mode, _update_swing_mode, _update_hvac_action) the branch only emits _LOGGER.error("Received invalid ..."), so this suppresses a spurious error when the source is merely unavailable — the same intent as the existing sentinels.

No behaviour change when templates render normally.

Notes

• Scoped deliberately to None rather than switching to a positive isinstance(temp, (int, float, str)) check, to keep the change minimal.
• Based on main, independent of Update make_template_entity_common_schema import #150.

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https://claude.ai/code/session_01R6qHpuFJDAavAGsfB4fvXm