Parse empty first line in msearch request body as action metadata

[javanna]

It looks like when parsing the msearch request body, we allow for the first line to be empty, yet that causes some different treatment for the first line that ends up requiring the action metadata line, while all other lines don't as they can be just empty.

With this change we properly accept the following which we would otherwise reject due to the first line being empty:

{ "query": {"match_all": {}}}

{ "query": {"match_all": {}}}

but we stop accepting the following (note the empty line before the first action metadata line:

{}
{ "query": {"match_all": {}}}

{ "query": {"match_all": {}}}

Relates to #39841

[elasticmachine]

Pinging @elastic/es-search

[cbuescher]

LGTM, I think failing on the second example (empty line and then an empty header line) is something to be expected

[javanna]

@jimczi do you have thoughts on whether this should be considered a breaking change, hence go to master only, or not?

[jimczi]

If it's a breaking change we also need to deprecate this format in 7x so would it be possible to log a deprecation warning if the first line is empty in 7x ? It's not a major bug so I guess it's ok to fix only in 8 and warn users about the deprecated format in 7.1 ?

[javanna]

ok @jimczi I did not manage to have both behaviours work at the same time, but I opened #41442 for the deprecation, which should be enough to push the "breaking" change to master.