fix(datasetToBlob): Allow dataset._meta to be omitted

[tsondergaard]

Without this it is necessary to add report.dataset._meta = {} before you can write a derived dataset like StructuredReport to part10 format:

const obj = await readFile("obj.dcm");
const dicom_data = dcmjs.data.DicomMessage.readFile(obj.buffer);
const report = new dcmjs.derivations.StructuredReport([dicom_data]);
report.dataset._meta = {}; // <-- Fix the need for this!
const buffer = dcmjs.data.datasetToDict(report.dataset).write();

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