[baremetal] prx: Fix empty Apache module list

[baremetal] prx: Fix empty Apache module list

On low state expansion, apache.server.modules is iterated, and
since reclass transforms '~' into 'null', an exception is thrown.

Use an empty string instead (different type will still ensure reclass
overrides variables from apache service instead of merging the lists).

While at it, add missing version 'latest' for mod-wsgi pkg.

Change-Id: I7913483b6a1be545b96de2958c39840940d9c557
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
(cherry picked from commit 40f907255380dbfc4d808d9f3c768db80c4c8158)