Fix VERSION when being zero.

This fixes a regression in all variables being reset if one is zero instead of not defined
-- COVESA DLT Viewer version: 2.29.0
-- COVESA DLT Viewer version: 0.0.0

With this fix:

-- COVESA DLT Viewer version: 2.29.0
-- COVESA DLT Viewer version: 2.29.0

Author: Gianfranco Costamagna

CMakeLists.txt

@@ -19,8 +19,7 @@ cmake_minimum_required (VERSION 3.15)
 include(scripts/windows/version.cmake)
 include(scripts/linux/version.cmake)
 
-message(STATUS "COVESA DLT Viewer version: ${DLT_PROJECT_VERSION_MAJOR}.${DLT_PROJECT_VERSION_MINOR}.${DLT_PROJECT_VERSION_PATCH}")
-if(NOT DLT_PROJECT_VERSION_MAJOR OR NOT DLT_PROJECT_VERSION_MINOR OR NOT DLT_PROJECT_VERSION_PATCH)
+if(NOT DEFINED DLT_PROJECT_VERSION_MAJOR OR NOT DEFINED DLT_PROJECT_VERSION_MINOR OR NOT DEFINED DLT_PROJECT_VERSION_PATCH)
     set(DLT_PROJECT_VERSION_MAJOR 0)
     set(DLT_PROJECT_VERSION_MINOR 0)
     set(DLT_PROJECT_VERSION_PATCH 0)