yaml: fix surgical Patch() for the empty-streams config round-trip

[firepinn]

Problem

go2rtc edits its YAML config in place with the surgical yaml.Patch() (used by the PATCH /api/config and streams API paths) rather than re-marshaling the whole document. Two edge cases in that patcher break the common "delete the last stream, then add one back" round-trip:

1. Deleting a path whose parent doesn't exist returns an error. addToEnd returns a fresh errors.New("yaml: path not exist"), which PatchConfig surfaces as a failure. A caller that deletes a possibly-absent key (e.g. removing a temporary stream) can't distinguish this from a real error.

2. Adding a child under an explicit null scalar fails to parse. yaml.Marshal of an emptied map emits the explicit token key: null. Patch() then pastes the new child mapping after that scalar, producing key: null\n child: ..., which is invalid YAML (mapping values are not allowed in this context). The patch is rejected (the API returns HTTP 400) and the key can't get its first child back without resetting the file.

Together these mean: delete every stream → the streams map serializes to streams: null → adding a stream back fails.

Fix

• Export the sentinel yaml.ErrPathNotExist and have PatchConfig treat a delete (value == nil) of a non-existent path as a no-op via errors.Is, instead of returning an error.
• When the parent key carries an explicit null scalar on its own line, strip that scalar so the key becomes a bare mapping parent before appending the child. A bare/implicit-null value (key:) is unaffected.

Tests

Added unit tests in pkg/yaml/yaml_test.go:

• deleting a path with an absent parent returns ErrPathNotExist;
• adding a child under key: null, both alongside a sibling key and on its own.

go test ./pkg/yaml/ passes.