Fix warning for buildXml method

Fix "Warning: Missing argument 3" for XmlEncoder::buildXml() called several times in XmlEncoder::selectNodeType()
merged branch fabpot/moved_serializer_4 (PR #7292)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #7292).

Commits
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4ffe234 Fix warning for buildXml method

Discussion
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Fix missing argument

This PR was submitted on the symfony/Serializer read-only repository and moved automatically to the main Symfony repository (closes symfony/Serializer#4).

Fix "Warning: Missing argument 3" for XmlEncoder::buildXml() called several times in XmlEncoder::selectNodeType()