Update SwiftmailerExtension.php

Optimize condition, it's more proper and clear this way.
minor #85 Use Yoda condition in SwiftmailerExtension (NAYZO)

This PR was merged into the 2.3-dev branch.

Discussion
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Use Yoda condition in SwiftmailerExtension

Optimize condition, it's more proper and clear this way.

Commits
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bc3a623 Update SwiftmailerExtension.php