Make bool query parsing stricter

[cbuescher]

A recent internal refactoring or our bool query parsing logic accidentally broke some edge case behaviour around ignoring null values
for several parameters like minimum_should_match or must, must_not, should or filter clauses were up until 7.7 we accepted queries like:

{
  "query": {
    "bool" : {
      "must" : null,
      "filter": {
        "term" : { "tag" : "tech" }
      }
  }
}

and ignored the null part. Although this usage seems to be rare, it lead to some issues so we restored the old behaviour. This is a follow up to discuss whether we want to remove this in 8.0 as a breaking change.

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[cbuescher]

Discussed and decided we don't need to remove supporting "null" really.