ParaViewInstallCMakePackageHelpers: set falsey variables locally

This follows the pattern used in
vtk/vtk@14cbef38.

CMake/ParaViewInstallCMakePackageHelpers.cmake

@@ -9,6 +9,8 @@ function (_paraview_package_append_variables)
       "if (NOT DEFINED \"${var}\")
   set(\"${var}\" \"${${var}}\")
   list(APPEND _paraview_find_package_variables \"${var}\")
+elseif (NOT ${var})
+  set(\"${var}\" \"${${var}}\")
 endif ()
 ")
   endforeach ()