pve7to8: content-dirs check: skip paths that cannot be resolved

pve7to8: content-dirs check: skip paths that cannot be resolved

The current inequality check for content-dirs does not correctly
handle the case in which `abs_path` returns undef. This can result in
confusing warnings:

  storage [...] uses directory  for multiple content types [...]

Fix this by skipping paths for which `abs_path` returns undef. This
matches the behavior of the actual content-dirs check in PVE 8 [0].

[0]: https://git.proxmox.com/?p=pve-storage.git;a=commit;h=09f1f847a

Fixes: ea0a4f1943ffafe94282afc800d5720db45df198
Signed-off-by: Friedrich Weber <f.weber@proxmox.com>