VariablesSanitizer mapValue if null

[HyunsangHan]

Hi!

I'm using apollo-tracing-uploader with Kotlin.

In Kotlin, I can set sanitizeVariables like below:

.sanitizeVariables(VariablesSanitizer.mapValues { it })

The error come out when the value, it in mapValues is null.
Is there any way to upload null value without any problem?