What does the JSON type Null mean in the GWAS REST API?

[ramiromagno]

Could you please clarify what type Null means exactly?

In the docs we see often that some objects have type Null, e.g., qualifier under retrieve a study.

While reading about the null type in json-schema.org it says that:

When a schema specifies a type of null, it has only one acceptable value: null.

In other places I read that it could mean missing data/not available.

So what exactly is its meaning in the JSON returned by the GWAS REST API?