[Users] prefs: Fix first block not being written on empty file

[Olivier Brunel]

Hey,

Playing with claws after having removed the clawsrc file, I noticed that
some preferences would seem to be "lost" the first time -- turns out
settings were written outside of any block, because when opening the
file failed (e.g. cause there's none) the block label wasn't being
written to the new file.

This should fix it.

Cheers,
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