Raise error if deps not provided with heads

[polm]

Before this change, if heads were passed without deps they would be
silently ignored, which could be confusing. See #8334.

Description

Types of change

Minor bugfix

Checklist

• I have submitted the spaCy Contributor Agreement.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[adrianeboyd]

It would be better to use a non-empty string here, and we normally use "dep" for a placeholder non-empty dep.

The problem with the head attribute is that there's no value for a missing head, so in Doc.has_annotation and other places, whether the dep is set is used to decide whether both heads and deps are set. So even if this works for the Doc creation (and I'm not sure it does in the end?), in other places it still won't look like there's a dependency parse.

[adrianeboyd]

I think an error as Matt suggested would be fine as an alternative to this. You could give a suggestion about how to fill in default deps as deps=["dep"]*len(heads) or something like that in the error message.

[polm]

Huh, turns out there's more than few tests that pass heads without deps.

[adrianeboyd]

Looks fine, I will just do some minor whitespace cleanup.