left and tree in MST nodes are documented as "optional" but implemented as "nullable"

[str4d]

The documentation:

atproto-website/content/specs/repository.md

Lines 87 to 92 in 0a7e942

	- `l` ("left", CID link, optional): link to sub-tree Node on a lower level and with all keys sorting before keys at this node
	- `e` ("entries", array of objects, required): ordered list of TreeEntry objects
	- `p` ("prefixlen", integer, required): count of bytes shared with previous TreeEntry in this Node (if any)
	- `k` ("keysuffix", byte array, required): remainder of key for this TreeEntry, after "prefixlen" have been removed
	- `v` ("value", CID Link, required): link to the record data (CBOR) for this entry
	- `t` ("tree", CID Link, optional): link to a sub-tree Node at a lower level which has keys sorting after this TreeEntry's key (to the "right"), but before the next TreeEntry's key in this Node (if any)

The implementation:
https://github.com/bluesky-social/atproto/blob/4184a652225eaf2de5f4d4702562e84c2fd3fde7/packages/repo/src/mst/mst.ts#L46-L56

const subTreePointer = z.nullable(common.cid)
const treeEntry = z.object({
  p: z.number(), // prefix count of ascii chars that this key shares with the prev key
  k: common.bytes, // the rest of the key outside the shared prefix
  v: common.cid, // value
  t: subTreePointer, // next subtree (to the right of leaf)
})
const nodeData = z.object({
  l: subTreePointer, // left-most subtree
  e: z.array(treeEntry), //entries
})

"nullable" is used elsewhere in the same document, and the corresponding implementation in that case matches (although with different syntax that I haven't yet traced):

atproto-website/content/specs/repository.md

Line 52 in 0a7e942

- `prev` (CID link, nullable): pointer (by hash) to a previous commit object for this repository. Could be used to create a chain of history, but largely unused (included for v2 backwards compatibility). In version `3` repos, this field must exist in the CBOR object, but is virtually always `null`. NOTE: previously specified as nullable and optional, but this caused interoperability issues.

https://github.com/bluesky-social/atproto/blob/4184a652225eaf2de5f4d4702562e84c2fd3fde7/packages/repo/src/types.ts#L17-L18

  // `prev` added for backwards compatibility with v2, no requirement of keeping around history
  prev: common.cid.nullable(),

[bnewbold]

I think you are probably right and these are nullable, but will double-check next week before updating the specs.