mgr/dashboard: fix subvolume group corruption from smb share form

mgr/dashboard: fix subvolume group corruption from smb share form

the SMB share form accidentally corrupts the subvolume group when it
issues a call to the subvolume_info API with an empty subvol_name which
then corrupts the group entirely and the following subvolume creation
gets failed.

The fix is to not call subvol_info with an empty name.

Fixes: https://tracker.ceph.com/issues/75771
Signed-off-by: Nizamudeen A <nia@redhat.com>