anchor.diff:  `field`::
   Name of the field you wish to search.
   +
  -To return a document, this field must exist and contain a value other
  -than `null` or `[]`. These values can include:
  +While a field is deemed non-existant if the JSON value is `null` or `[]`, these values will indicate the field does exist:
   +
   * Empty strings, such as `""` or `"-"`
   * Arrays containing `null` and another value, such as `[null, "foo"]`
  
positive.code: ==== Notes
  
  [[find-docs-null-values]]
  ===== Find documents with null values
  To find documents that contain only `null` values or `[]` in a provided field,
  use the `must_not` <<query-dsl-bool-query, boolean query>> with the `exists`
  query.
  
  The following search returns documents that contain only `null` values or `[]`
  in the `user` field.
  
  [source,js]
  ----
  
negative.code: [[query-dsl-exists-query]]
  === Exists Query
  
  Returns documents that contain an indexed value for a field.
  
  An indexed value may not exist for a document's field due to a variety of reasons:
  
  * The field in the source JSON is `null` or `[]`
  * The field has `"index" : false` set in the mapping
  * The length of the field value exceeded an `ignore_above` setting in the mapping
  * The field value was malformed and `ignore_malformed` was defined in the mapping
  
  [[exists-query-ex-request]]
  ==== Example request
  