add restx initialization; modified deserialization to check empty fields

[Litou-lyh]

Modified files:

• service/__init__.py: added restx api initialization
• models/wishlist.py: added checks for empty fields
• models/wishlist_item.py: added checks for empty fields

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files	Coverage Δ
service/__init__.py	92.00% <100.00%> (+1.09%)	:arrow_up:
service/models/wishlist.py	97.82% <100.00%> (+0.20%)	:arrow_up:
service/models/wishlist_item.py	97.77% <100.00%> (+0.21%)	:arrow_up:
service/routes.py	98.91% <100.00%> (+0.03%)	:arrow_up:

... and 1 file with indirect coverage changes