Fix crash of xmlconcat(NULL)

Peter Eisentraut <peter_e@gmx.net>

Commit: 05798c9275261c1f9308dea5e1bee559e461b689
Author: Peter Eisentraut <peter_e@gmx.net>
Date: 2008-11-15T20:53:40Z
Releases: 8.3.6
Fix crash of xmlconcat(NULL)

backpatch from 8.4devel

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