Resolving wildcards in aliases expression is challenging as we may end
up with no aliases to replace the original expression with, but if we
replace with an empty array that means _all which is quite the opposite.
Now that we support and serialize the original requested aliases,
whenever aliases are replaced we will be able to know what was
initially requested. MetaData#findAliases can then be updated to not
return anything in case it gets empty aliases, but the original aliases
were not empty. That means that empty aliases are interpreted as _all
only if they were originally requested that way.

Relates to elastic#31516

this change is breaking, will not be backported to 6.x