Fix: check if compression_map format is undefined

Fix: check if compression_map format is undefined

We want to check for an supported compression type, but the check was
not correct as this only works if both sides are scalars, but an
assignment to an array is always "truthy", so actually check explicitly
if the compression type is supported before.

Signed-off-by: Alwin Antreich <a.antreich@proxmox.com>
Co-authored-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
(cherry picked from commit 539660e2bd3e9a557a8c0ce7e17865377bf269b9)
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>