fix(NEP): Return "Null" as ClassName of null

- AbstractVariablesResource#getVariables() does not throw a
  NullPointerException anymore, when the variable value is. In that case
  the simple class name will be set to "Null"

related to CAM-1020
fix(typo): Fix typo in engine selectbox
Merge branch 'master' of github.com:camunda/camunda-bpm-platform