fix handing of MANUALLY_ADDED_DEPENDENCIES

CMake/cotire.cmake

@@ -3121,6 +3121,7 @@ function (cotire_collect_unity_target_sources _target _languages _unityTargetSou
 			list (APPEND _unityTargetSources ${_unityFiles})
 		endif()
 	endforeach()
+	# handle object libraries which are part of the target's sources
 	get_target_property(_linkLibrariesStrategy ${_target} COTIRE_UNITY_LINK_LIBRARIES_INIT)
 	if ("${_linkLibrariesStrategy}" MATCHES "^COPY_UNITY$")
 		cotire_filter_object_libraries(${_target} _objectLibraries ${_targetSourceFiles})
@@ -3469,7 +3470,9 @@ function (cotire_target_link_libraries _target)
 			get_target_property(_manualDependencies ${_target} MANUALLY_ADDED_DEPENDENCIES)
 			if (_manualDependencies)
 				cotire_map_libraries("${_linkLibrariesStrategy}" _unityManualDependencies ${_manualDependencies})
-				add_dependencies("${_unityTargetName}" _unityManualDependencies)
+				if (_unityManualDependencies)
+					add_dependencies("${_unityTargetName}" ${_unityManualDependencies})
+				endif()
 			endif()
 		endif()
 	endif()