fix: remove <null> from Value to fix TopLevelSpec consumed by code using strict mode

[domoritz]

Copy of #7352

[walterra]

Looks like all tests passed! 🥳

Great if there are no changes! Before and after, is there any externally observable change? Can you assign a value that could not be assigned before or vice versa?

The generated spec didn't change and since null was already part of the allowed values as part of PrimitiveValue there should also be no change in how to use the TS interface.

[domoritz]

That is not correct.

const v: VgValueRef = {
  value: {signal: 'string'}
};

used to not be allowed and now is