libdm: report: make it possible to use blank value as selection for string list report field

$ lvs -o name,tags vg
  LV    LV Tags
  lvol0
  lvol1 mytag

Before this patch:
$ lvs -o name,tags vg -S 'tags=""'
  Failed to parse string list value for selection field lv_tags.
  Selection syntax error at 'tags=""'.
  Use 'help' for selection to get more help.

(and the same for -S 'tags={}' and -S 'tags=[]')

With this patch applied:
$ lvs -o name,tags vg -S 'tags=""'
  LV    LV Tags
  lvol0

(and the same for -S 'tags={}' and -S 'tags=[]')
3 files changed