Make fields optional on multi_match query

[russcam]

Rely on index.query.default_field by default when no fields specified

Remove Fields from the conditionless check

elasticsearch-net/src/Nest/QueryDsl/FullText/MultiMatch/MultiMatchQuery.cs

Line 97 in 23ae899

internal static bool IsConditionless(IMultiMatchQuery q) => q.Fields.IsConditionless() || q.Query.IsNullOrEmpty();