AIX: Restore pre-3.16 undocumented method to suppress exports with XL

In commit 0f150b69d3 (AIX: Explicitly compute shared object exports for
both XL and GNU, 2019-07-11, v3.16.0-rc1~418^2~2) we dropped use of the
old `CMAKE_XL_CreateExportList` cache entry for XL exports.  However,
some people were setting the value to an empty string as a way to
disable automatic export of symbols.  Restore this behavior when the
option is explicitly set to an empty string.

Issue: #20290

Modules/Platform/AIX-XL.cmake

@@ -18,14 +18,24 @@ macro(__aix_compiler_xl lang)
 
   set(CMAKE_${lang}_LINK_FLAGS "-Wl,-bnoipath")
 
+  set(_OBJECTS " <OBJECTS>")
+  if(DEFINED CMAKE_XL_CreateExportList AND CMAKE_XL_CreateExportList STREQUAL "")
+    # Prior to CMake 3.16, CMAKE_XL_CreateExportList held the path to the XL CreateExportList tool.
+    # Users could set it to an empty value to skip automatic exports in favor of manual -bE: flags.
+    # Preserve that behavior for compatibility (even though it was undocumented).
+    set(_OBJECTS "")
+  endif()
+
   # Construct the export list ourselves to pass only the object files so
   # that we export only the symbols actually provided by the sources.
   set(CMAKE_${lang}_CREATE_SHARED_LIBRARY
-    "\"${CMAKE_ROOT}/Modules/Platform/AIX/ExportImportList\" -o <OBJECT_DIR>/objects.exp <OBJECTS>"
+    "\"${CMAKE_ROOT}/Modules/Platform/AIX/ExportImportList\" -o <OBJECT_DIR>/objects.exp${_OBJECTS}"
     "<CMAKE_${lang}_COMPILER> <CMAKE_SHARED_LIBRARY_${lang}_FLAGS> -Wl,-bE:<OBJECT_DIR>/objects.exp <LANGUAGE_COMPILE_FLAGS> <LINK_FLAGS> <CMAKE_SHARED_LIBRARY_CREATE_${lang}_FLAGS> <SONAME_FLAG><TARGET_SONAME> -o <TARGET> <OBJECTS> <LINK_LIBRARIES>"
     )
 
   set(CMAKE_${lang}_LINK_EXECUTABLE_WITH_EXPORTS
     "\"${CMAKE_ROOT}/Modules/Platform/AIX/ExportImportList\" -o <TARGET_IMPLIB> -l . <OBJECTS>"
     "<CMAKE_${lang}_COMPILER> <FLAGS> <CMAKE_${lang}_LINK_FLAGS> -Wl,-bE:<TARGET_IMPLIB> <LINK_FLAGS> <OBJECTS> -o <TARGET> <LINK_LIBRARIES>")
+
+  unset(_OBJECTS)
 endmacro()