Uninitialized DH computeSecret should throw instead of returning empty buffers

When a DH instance does not have a private key initialized, it just happily returns empty buffers for all public keys.

That is a footgun and needs to be fixed (turned into a throw).

```js
Welcome to Node.js v26.2.0.
Type ".help" for more information.
> require('crypto').createDiffieHellman(512).computeSecret(Buffer.alloc(64,1))
<Buffer >
```

See also #62838 