Missing default value for TC for schema validation

[Alival-IT]

@nodh

if you scan a code with missing test facility, the validation fails. the fix is to define the default value as null

@SerialName("tc")
val testFacility: String? = null,

i think the default values are needed, or kotlinx serialization can enable the null validation/parsing

you can test it here
https://github.com/eu-digital-green-certificates/dcc-quality-assurance/blob/main/SI/1.3.0/specialcases/TEST-AG.png