[th/strv-cleanup] minor cleanups related to strv handlng
- glib-aux: add nm_strv_dup_full() helper to preserve empty strv
- all: use nm_strv_empty_new() helper
- glib-aux: add nm_strv_dup_full() helper to preserve empty strv
- glib-aux: add nm_strv_ptrarray_to_strv() helper
- libnm: drop unused nm_strv_to_ptrarray() helper
- libnm: replace _nm_utils_ptrarray_to_strv() with nm_strv_ptrarray_to_strv()