Cannot read server-sent events without data (a.k.a. heartbeats)

[BvanMunster]

Affects: 5.3.11+
org.springframework.http.codec.ServerSentEventHttpMessageReader.buildEvent(List<String> lines, ResolvableType valueType, boolean shouldWrap, Map<String, Object> hints) yields a StringIndexOutOfBoundsException if a line equals "data:", because line.charAt(5) is called.
This means that it fails at reading server-sent events that are heartbeats.

[jhoeller]

This seems to have been caused by #27473, fine-tuning the removal of spaces in SSE data.