fix #168 empty tuple used for Sequence type

[michalchrobok]

Fix proposal for #168

Changes:

• is_instance checks tuple values types only if dataclass type hint is actually Tuple
• _build_value_for_collection returns () for empty tuple instead of (None,)

[mcgibbon]

@konradhalas @antonagestam is this ready to merge now?

[konradhalas]

@michalchrobok thank you! I had to fix some small issues, so its merged manually via https://github.com/konradhalas/dacite/commit/a636b37106cfc95a3fa402706fb6cb768dbab3d5