sync() should handle flow sequences gracefully

[nathanjmcdougall]

Currently, sync() raises PatchError when trying to insert into a flow sequence (e.g. repos: []). This forces callers to wrap sync() in a try/except and fall back to upsert():

try:
    doc = doc.sync("repos", value=repos_list)
except yamltrip.PatchError:
    doc = doc.upsert("repos", value=repos_list)

Proposal: When sync() needs to insert into an empty flow sequence, it should convert the node to a block sequence automatically. An empty flow sequence ([]) has no formatting worth preserving, so this is always the correct thing to do.

For non-empty flow sequences where patching fails, falling back to a full replacement (like upsert) internally would also be reasonable.

This would let callers use sync() unconditionally without needing to handle PatchError.