lib: Use NULLSTR family of macros more

lib: Use NULLSTR family of macros more

There is a family of convenient macros: NULLSTR, NULLSTR_EMPTY,
NULLSTR_STAR, NULLSTR_MINUS which hides ternary operator.

Generated using the following spatch (and its obvious variants):

  @@
  expression s;
  @@
    <+...
  - s ? s : "<null>"
  + NULLSTR(s)
    ...+>

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>