Translation Manager - RTE empty-markup crash fix

Translation Manager: fixed a crash where an RTE property with empty markup (e.g. a block-only rich text value) came back from translation as {"markup": null, ...} instead of {"markup": "", ...}, making the node unopenable in the backoffice
Fixed on both v17 (v17.7.1) and v18 (v18.1.1), same root cause in HtmlDocumentMapper.GetSimpleTargetValue
Published v18.1.1 and v17.7.1 release notes, both to NuGet