private void checkThreeNans(Tensor t) {
        final Double nan = Double.NaN;
        int cnt = 0;
        for (var iter = t.cellIterator(); iter.hasNext(); ) {
            var cell = iter.next();
            assertEquals(nan, cell.getValue());
            ++cnt;
        }
        assertEquals(3, cnt);
    }
All cell types are rendered as double.
vespajlib/src/test/java/com/yahoo/tensor/serialization/JsonFormatTestCase.java