fix: Return global subject if there is no personalization

[geryjuhasz]

Fixes

Fixes #813

Checklist

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• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
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Short description of what this PR does:

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[childish-sambino]

This change doesn't take into account setting the subject before any personalizations are added. Using the OPs example:

$mail = new \SendGrid\Mail\Mail();
$mail->setSubject('my subject');
$mail->getSubject();

The returned subject is still null.