fixed NPE in DebugStringVariableSubstitutor when var cannot be resolved

Change-Id: I5fe8f640e6dbf1673d32d6dec14aa2f9190a71de
Signed-off-by: Alena Laskavaia <elaskavaia.cdt@gmail.com>
Reviewed-on: https://git.eclipse.org/r/36405
Tested-by: Hudson CI
Reviewed-by: Sergey Prigogin <eclipse.sprigogin@gmail.com>