[Property Editor] TextDocumentIdentifier.version might be null

[elliette]

In order to support IntelliJ (which currently does not send a text document version identifier) we have decided to make it optional (see comment at https://dart-review.googlesource.com/c/sdk/+/392908/comment/dd68c796_bd6c34ef/).

The API class will need to be updated to handle the nullable value:

devtools/packages/devtools_app/lib/src/service/editor/api_classes.dart

Line 348 in f7a9352

final int version;