add generator to tsvector type

[jsw800]

My team uses the Tsvector type, and we're trying to start using generators in tests, but tsvector currently doesn't have a generator so trying to generate something with it raises. This adds one that just always creates an empty list. I'm open to suggestions if you think the generator should be doing something more complex here—I assume this is a scenario where, if someone wants to be generating something specific for it in a test, they probably have a clear idea of what that is.

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[zachdaniel]

This is reasonable for now, doesn't hurt, but we may eventually want to figure out some way to do it in a more realistic way etc.