nil slice to zero-length blob

[lucasmrod]

Changes to fix #424 also make nil slice be converted to zero-length blob. Instead, nil slice should be converted to value NULL.

Current code:
	[]byte -> zero length blob
	nil    -> zero length blob
Proposal:
	[]byte -> zero length blob
	nil    -> NULL blob

I can submit a PR if the proposal makes sense to you.

[gjrtimmer]

@lucasmrod makes sense, can you submit a PR of evaluation and testing, please also include tests in your PR.