use NULL_RLP, remove NULL_RLP_STATIC

[guanqun]

I thought NULL_RLP_STATIC was created for a reason, but I can't think of the specific reason, even the Rust book suggests we should prefer const. So this PR, but maybe it has other reasons, then feel free to close this PR.

cargo build and cargo test both seem ok.

[rphmeier]

LGTM, still working on getting CI up for out-of-org PRs! Thanks for your patience and repeated contributions :)

[guanqun]

Thank you all to come up with this great project. For me, I'm only able to work on this part time (limited time per day), so bear with me to create these small/trivial PRs. Hopefully when I get more familiar with the whole codebase, I can send bigger PR.