ML API procedures handle null values being passed in better

[jexp]

If you construct a string and one of the values (e.g. movie.plot) is null the whole string will be null, which currently causes the api call to also fail with a 400 error, that’s why currently one has to use coalesce functions.

But we should probably just filter those out in the embeddings call and don't forward them and return the result row with a null value for embedding.

For the other ml api calls, we might just want to return a null value for a null input or create an exception upfront so that the user knows what went wrong.

Should probably also look into better exposing http error messages in the loadJson() function, currently they are not passed along in the error message.