mkconf: fix SOURCE_DATE_EPOCH variable usage

In commit 04adb527, support for SOURCE_DATE_EPOCH got introduced. Using "[ -z $SOURCE_DATE_EPOCH ]" in mkfconf doesn't work though, as the script is running under set -u, and the variable isn't set by default, and then fails with:

/usr/share/mdadm/mkconf: 98: SOURCE_DATE_EPOCH: parameter not set

Closes: #992845

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