Bluetooth: GOEP: Support the empty Name header case

According to the OBEX Version 1.5, the Name header could be a empty
string.

Update function `bt_obex_add_header_name` to support the case that the
length of name could be 0.

Update function `bt_obex_get_header_name` to support the case that the
name header is found but the name length is 0.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>