Fix: Silently ignore empty JSON string object for MQTT IR signal payload

[jbouwh]

Proposed change

Fix: Silently ignore empty JSON string object for MQTT IR signal payload. When the infrared MQTT entity receives timings, these are often extracted using a valuetemplate. While a dict is expected we should ignore an empty JSON string "".
This fix wil no longer log warnings when the value is an empty string, like:

Invalid message received for infrared.irhub_learned_ir_timings on topic zigbee2mqtt/irhub, with template Template<template=({{ value_json.learned_ir_timings | tojson }}) renders=1>. Message is not a valid signal JSON message. Got

{"ir_code_to_send":null,"ir_emitter":null,"learn_ir_code":null,"learned_ir_code":"Some code...","learned_ir_timings":"","linkquality":120}

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• This PR fixes or closes issue: fixes #
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[home-assistant]

Hey there @emontnemery, @bdraco, mind taking a look at this pull request as it has been labeled with an integration (mqtt) you are listed as a code owner for? Thanks!

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[Copilot]

Pull request overview

Silently ignores empty JSON-string MQTT infrared payloads instead of logging warnings.

Changes:

• Treats "" as an ignorable payload.
• Adds regression coverage for debug logging and signal suppression.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
homeassistant/components/mqtt/infrared.py	Ignores empty JSON-string payloads.
tests/components/mqtt/test_infrared.py	Tests the new behavior.