Parses a single SSE data: line and returns a typed result.
data:
A raw SSE line, e.g. "data: {...}" or "data: [DONE]".
"data: {...}"
"data: [DONE]"
null if the line is not a data: line (e.g. comment, empty).
null
Parses a single SSE
data:line and returns a typed result.