fix #2374: bridge-ports is assumed to be defined

fix #2374: bridge-ports is assumed to be defined

bridge-ports is an optional parameter but was assumed to be defined in
the code. This resulted in 'use of uninitialized value' warnings.

By reordering of the $ports definition before the replace, we get rid of
the 'use of uninitialized value' when no bridge-port is specified.

In the second case we set it to an empty string when it is undefined.

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>