fix(api): reject empty dense vectors in gRPC Vector::Dense validation - #10143

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What does this PR do?

grpc::vector::Vector::Dense was the only variant of the gRPC Vector oneof that skipped emptiness validation:

impl Validate for grpc::vector::Vector {
    fn validate(&self) -> Result<(), ValidationErrors> {
        match self {
            grpc::vector::Vector::Dense(_dense) => Ok(()),   // no check
            grpc::vector::Vector::Sparse(sparse) => sparse.validate(),
            grpc::vector::Vector::MultiDense(multi) => multi.validate(),
            ...
        }
    }
}