Specify a value can be a string or null with JSON Schema

From https://json-schema.org/understanding-json-schema/reference/type.html

The type keyword may either be a string or an array:

• If it’s a string, it is the name of one of the basic types above.
• If it is an array, it must be an array of strings, where each string is the name of one of the basic types, and each element is unique. In this case, the JSON snippet is valid if it matches any of the given types

The same page lists also the defined data type names, including string and null.

Try:

"member_region": { "type": ["string", "null"] }