CMake: Handle empty INCLUDE_DIRECTORIES in genex for qdoc

Follow the pattern recommended in the CMake docs for the $<JOIN:...>
generator expression. Wrap it in a $<BOOL:...> genex so that if a
target's INCLUDE_DIRECTORIES property is empty, we don't add a
stray -I with no directory following it. This fixes the following error
when running qdoc:

  qdoc: Missing value after '-I'.

Pick-to: 6.0
Task-number: QTBUG-88838
Change-Id: Ie3ef2625fbb29cc501f7fc22ff7a9cc8ac548322
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtDocsHelpers.cmake

@@ -65,7 +65,7 @@ function(qt_internal_add_docs)
     if (NOT target_type STREQUAL "UTILITY")
         file(GENERATE
             OUTPUT ${target_include_dirs_file}
-            CONTENT "-I$<JOIN:${include_paths_property},\n-I>"
+            CONTENT "$<$<BOOL:${include_paths_property}>:-I$<JOIN:${include_paths_property},\n-I>>"
         )
         set(include_path_args "@${target_include_dirs_file}")
     else()