Forbid empty doc values on vector functions

[mayya-sharipova]

Currently when a document misses a vector value, vector function
returns 0 as a score for this document. We think this is incorrect
and unexpected behaviour.
With this change, an error will be thrown if vector functions are
used with docs that are missing vector doc values.
Also VectorScriptDocValues is modified to allow size() function,
which can be used to check if a document has a value for the
vector field.

[elasticmachine]

Pinging @elastic/es-search

[mayya-sharipova]

@jpountz I wanted to ask if you are fine if we expose size() function in VectorScriptDocValues?

Before we made a decision not to leak the internal representations of vectors.
But after thinking about it with @jtibshirani, we think that we need to expose size() function which will allow users to check if a document has a value for a vector field.

[jpountz]

To be clear, my comment in the other PR was about the fact that users could have access to the binary representation of the vector. Exposing size() as a way to know whether the document has a value or not sounds good to me. 👍

[jtibshirani]

This also looks good to me.

[jtibshirani]

Small comment: now that you can call size on a vector field, perhaps we could update the error message above to say "accessing a vector field's value through 'get' or 'value' is not supported".