You can import NULL values using JSON format source files - omit the key:value pair for values that are NULL.
Example - Let's say you have a schema like this:
{
"name": "kind",
"type": "string"
},
{
"name": "fullName",
"type": "string",
},
{
"name": "age",
"type": "integer",
"mode": "nullable"
}
A record with no NULL values might look like this:
{"kind": "person",
"fullName": "Some Person",
"age": 22
}
However, when "age" is NULL, try this (note, no "age" key):
{"kind": "person",
"fullName": "Some Person",
}
Please let us know if you have issues with this. I'll make a note to improve the documentation around using NULL values with JSON import formats.