'lupdate_sources' variable empty in cmake file generated by qt_add_lupdate

The issue can be reproduced with Qt Creator and a CMake project.

• Use:

  set(TS_FILES bug_test_en.ts bug_test_de.ts bug_test_it.ts)
  qt_add_lupdate(bug_test TS_FILES ${TS_FILES})

  run CMake, go into the build directory and look into the .lupdate/bug_test_project.cmake file.
  You will find a row starting with

   set(lupdate_sources "...
  

  followed by all the sources associated to the executable by the qt_add_executable() call.

• Now suppose you want to limit the translation process to a subset of the sources; for example:

  set(SOURCES_A src_a_1.cpp src_a_1.h src_a_2.cpp src_a_2.h)
  set(SOURCES_B src_b_1.cpp src_b_1.h src_b_2.cpp src_b_2.h)
  qt_add_executable(bug_test ${SOURCES_A} ${SOURCES_B})
  qt_add_lupdate(${CMAKE_PROJECT_NAME} TS_FILES ${TS_FILES} SOURCES ${SOURCES_A})
  

  run CMake, go into the build directory and look into the .lupdate/bug_test_project.cmake file.
  You will find that the lupdate_sources variable is set to an empty string, like this

  set(lupdate_sources "")
  

  This makes the call to the update_translations target not working (all the existing translations turn to vanished).

NB: the INCLUDE_DIRECTORIES argument should suffer from the same issue.

A workaround (and suggested fixup) is:

1. locate the Qt6LinguistToolsMacros.cmake file in the Qt installation path
2. locate the function(qt6_add_lupdate target) definition (it should be at about line 148)
3. locate the block

       if(NOT arg_INCLUDE_DIRECTORIES)
           set(includePaths "$<TARGET_PROPERTY:${target},INCLUDE_DIRECTORIES>")
       endif()
       if(NOT arg_SOURCES)
           set(sources "$<TARGET_PROPERTY:${target},SOURCES>")
       endif()

   (it should be at about line 161) and turn it into

       if(NOT arg_INCLUDE_DIRECTORIES)
           set(includePaths "$<TARGET_PROPERTY:${target},INCLUDE_DIRECTORIES>")
       else()
           qt_internal_make_paths_absolute(includePaths "${arg_INCLUDE_DIRECTORIES}")
       endif()
       if(NOT arg_SOURCES)
           set(sources "$<TARGET_PROPERTY:${target},SOURCES>")
       else()
           qt_internal_make_paths_absolute(sources "${arg_SOURCES}")
       endif()