Steps to reproduce

Scenario 1)

• using a call activity
• passing a variable
• which is a Java Object serialized as JSON
  => in the sub process instance the variable is serialized using the default serialization format

Scenario 2)

• using a call activity
• passing a variable
• which is of type String and value 'null'
  => in the sub process instance the variable is of type 'Null'

Expected behavior:

• the variable is copied including all metadata

Hint:
Probably the variable copying does not use the typed api
the type information is not passed, see https://github.com/camunda/camunda-bpm-platform/blob/master/engine/src/main/java/org/camunda/bpm/engine/impl/core/model/CallableElement.java#L201