schema validation: don't fail on missing source if NoSourceSince is set

eg. https://gitlab.com/fdroid/fdroiddata/-/blob/d0a66b9162126ab6dc6b72e16329f515aedc99b3/metadata/cz.yetanotherview.webcamviewer.app.yml

fail: https://gitlab.com/fdroid/fdroiddata/-/jobs/3414297060#L7865

+ ajv validate -s schemas/metadata.json -d metadata/cz.yetanotherview.webcamviewer.app.yml
metadata/cz.yetanotherview.webcamviewer.app.yml invalid
[
  {
    instancePath: '',
    schemaPath: '#/required',
    keyword: 'required',
    params: { missingProperty: 'SourceCode' },
    message: "must have required property 'SourceCode'"
  }
]