Resolve PHP deprecation for json_decode

[bagene]

Description

This will resolve a PHP deprecation for json_decode() when Symfony\Component\Mime\Email::$html is null. This happen when using Illuminate\Support\Facades\Mail::raw() where $html is never set on the Email class.

Motivation and context

We're in the process of eliminating PHP deprecations in our codebase for safe PHP upgrades in the future.

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