uSync.Migrations: fixed RTE import crash, released v17.0.3

Repo: Jumoo/uSyncMigrations

Fixed MacroRTEConfigSerializer throwing JsonException on datatypes whose blocks config serialized to an empty/whitespace string (Umbraco.TinyMCE, stock RichtextEditor.config) — this was aborting every RTE datatype import and cascading into dropped RTE properties on content types (#330)
Backfilled a missing 17.0.2 changelog entry that had been sitting under "Unreleased"
Released v17.0.3 to NuGet