Rails will automatically strip empty arrays and hashes from params. This is to fix security holes caused by the unintentional generation of SQL queries with IS NULL or IN ('foo', NULL').

For example, all of the following JSON:

{ "foo": null }
{ "foo": [] }
{ "foo": [null] }
{ "foo": [null, null, ...] }

will be turned into this Ruby hash:

{ foo: nil }

nulls are also cleaned out of arrays. The following JSON:

{ "foo": ["bar", null] }

is parsed to to:

{ foo: ["bar"] }

Rails invokes a method called deep_munge to do this work. Check out the [rails security guide] for more details.

[rails security guide]: http://guides.rubyonrails.org/security.html#unsafe-query-generation