Bugfix: Ignore empty relationships on App Store Connect resources

[priitlatt]

It is possible that App Store Connect API returns resources with empty relationships. For example List Apps endpoint can respond with a response where an app has ciProduct relationship defined as {} instead of null even though the schema suggests that relationships should always have at least links attribute defined.

Such a response breaks data deserialization.

Example with stacktrace

def test_empty_relationship():
>       App(
            {
                "type": "apps",
                "id": "...",
                "links": {...},
                "attributes": {...},
                "relationships": {
                    ...,
                    "ciProduct": {},  # <-- empty relationship from App Store Connect API response!
                },
            },
        )

test_app_resource.py:18: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../../../src/codemagic/apple/resources/resource.py:241: in __init__
    self.relationships = self._create_relationships(api_response)
../../../src/codemagic/apple/resources/resource.py:233: in _create_relationships
    return cls.Relationships(**defined_fields)
<string>:18: in __init__
    ???
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = App.Relationships(appInfos=Relationship(links=<codemagic.apple.resources.resource.Links object at 0x106058680>, data=N..., perfPowerMetrics={'links': {'related': 'https://api.appstoreconnect.apple.com/v1/apps/1481211155/perfPowerMetrics'}})

    def __post_init__(self):
        for field in self.__dict__:
            value = getattr(self, field)
            if not isinstance(value, (Relationship, type(None))):
>               setattr(self, field, Relationship(**value))
E               TypeError: Relationship.__init__() missing 1 required positional argument: 'links'

../../../src/codemagic/apple/resources/resource.py:215: TypeError

Handle empty relationships by just discarding them.