libmbim-glib,tlv: allow 0-length strings for MBIM_TLV_TYPE_WCHAR_STR
Document Microsoft MBIM Extensions for 5G (rev 1.17) considers the possibility of having wchar strings tlvs with datalength == 0 in the information buffer (e.g. AccessString field in MBIM_CONNECT_INFO_EX3 data structure).
Modify mbim_tlv_string_get and related calls for dealing with these situations, returning an empty string.
Edited by Daniele Palmas