fix: city of a location can be null

[Rocksheep]

Description

Some locations will return an empty string for the city name. This results in the package to throw an error due to a non-nullable city property on Location.

Motivation and context

The package throws errors when the city property is empty. This PR resolves it.

How has this been tested?

I have ran a test importing the data. Some of the situations had an empty string. One of these is https://melvin.ndw.nu/melvinservice/rest/public/situation/385390

Running the integration tests will show that it now works again

Screenshots (if appropriate)

Types of changes

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[JaZo]

Please verify with NDW that this change is expected.