[CMake] Fixup Keras version check for tutorials

The Keras version check that was introduced in 2ddaee3 should only be
done if Keras is found. Otherwise, we get wrong warnings about how the
Keras version could not be determined

Author: guitargeek

tutorials/CMakeLists.txt

@@ -349,9 +349,9 @@ else()
   # Keras tutorials. This mirrors the logic in tmva/sofie/test/CMakeLists.txt.
   # TODO: make sure we also support the newest Keras
   set(unsupported_keras_version "3.5.0")
-  if (NOT DEFINED ROOT_KERAS_VERSION)
+  if (ROOT_KERAS_FOUND AND NOT DEFINED ROOT_KERAS_VERSION)
     message(WARNING "Keras found, but version unknown — cannot verify compatibility.")
-  elseif (NOT ROOT_KERAS_VERSION VERSION_LESS ${unsupported_keras_version})
+  elseif (ROOT_KERAS_FOUND AND NOT ROOT_KERAS_VERSION VERSION_LESS ${unsupported_keras_version})
       message(WARNING "Keras version ${ROOT_KERAS_VERSION} is too new for the SOFIE Keras parser (only supports < ${unsupported_keras_version}). Corresponding tutorials will not be tested.")
     set(keras_unsupported TRUE)
   endif()