Fix handling undefined INTERFACE_REQUIRED_PACKAGES

cmake/modules/LibraryTarget.cmake

@@ -200,8 +200,10 @@ if (META_HEADER_ONLY_LIB)
 endif ()
 
 # create the CMake package config file from template
-list(REMOVE_ITEM INTERFACE_REQUIRED_PACKAGES "")
-list(REMOVE_DUPLICATES INTERFACE_REQUIRED_PACKAGES)
+if (INTERFACE_REQUIRED_PACKAGES)
+    list(REMOVE_ITEM INTERFACE_REQUIRED_PACKAGES "")
+    list(REMOVE_DUPLICATES INTERFACE_REQUIRED_PACKAGES)
+endif ()
 set(CONFIG_TARGETS "${CMAKE_CURRENT_BINARY_DIR}/${META_PROJECT_NAME}${META_CONFIG_SUFFIX}Config.cmake")
 if (META_CONFIG_SUFFIX)
     list(APPEND CONFIG_TARGETS "${CMAKE_CURRENT_BINARY_DIR}/${META_PROJECT_NAME}Config.cmake")