FwLiteWeb creating new sense has entryId "" (empty string), which .NET refuses to parse as a GUID

[rmunn]

Describe the bug
Creating a new sense in an example FwLite project ends up with:

Error: System.Text.Json.JsonException: The JSON value could not be converted to System.Guid. Path: $.senses[0].entryId | LineNumber: 0 | BytePositionInLine: 196.

The JSON in question includes Senses[0] as { id: "00000000-0000-0000-0000-df85dbc25d08", entryId: "", definition: {…}, … } - note entryId being an empty string.

To Reproduce
Steps to reproduce the behavior:

1. Run task fw-lite-web
2. Create new example project
3. Create new sense in first entry
4. Edit definition field, tab out
5. See error

Expected behavior
A missing entryId would be serialized as null rather than an empty string, or the C# backend code would handle the empty string as a special case in deserializing GUIDs and replace it with either null (for a nullable GUID property) or Guid.Empty (for a non-nullable GUID property).