Trie: make NULL available as type option for put value

[ScottyPoi]

trie.put(key, null)
and
trie.put(key, Uint8Array.from([])
both should (and do) delete the value for that key:

ethereumjs-monorepo/packages/trie/src/trie.ts

Lines 194 to 198 in e2dc88b

	// If value is empty, delete
	if (value === null || value.length === 0) {
	return this.del(key)
	}

--

However, Null is left out of the type options for the value paremeter:
async put(key: Uint8Array, value: Uint8Array, skipKeyTransform: boolean = false): Promise<void>

The official trie tests actually do use null as a value. The reason the test does not throw a Type error is that the null values are wrapped in a variable with type any

Since PUT does not actually accept Null values outright, users are forced to use empty Uint8Arrays, or as any to access this part of the implementation.

---

This PR adds | null as a type options for the value parameter, bringing into alignment with the implementation itself.

[codecov]

Codecov Report

Merging #3020 (ffbe758) into master (8ca49a1) will decrease coverage by 2.72%.
The diff coverage is 100.00%.

Additional details and impacted files

Flag	Coverage Δ
block	88.73% <ø> (?)
blockchain	92.58% <ø> (?)
client	87.33% <ø> (+0.04%)	⬆️
common	98.18% <ø> (?)
ethash	∅ <ø> (∅)
evm	69.78% <ø> (?)
rlp	∅ <ø> (?)
statemanager	84.91% <ø> (?)
trie	90.75% <100.00%> (?)
tx	96.38% <ø> (ø)
util	86.78% <ø> (?)
vm	74.66% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

[scorbajio]

ACK