Merge pull request #808 from commontk/update-external-dependency-module

ENH: Update ExternalProjectDependency system based on commontk/Artichoke@0cc99d3

CMake/ctkMacroCheckExternalProjectDependency.cmake

@@ -458,7 +458,8 @@ function(_sb_get_external_project_arguments proj varname)
     get_property(${proj}_EP_PROPERTY_${property} GLOBAL PROPERTY ${proj}_EP_PROPERTY_${property})
     get_property(${_ALL_PROJECT_IDENTIFIER}_EP_PROPERTY_${property} GLOBAL PROPERTY ${_ALL_PROJECT_IDENTIFIER}_EP_PROPERTY_${property})
     set(_all ${${proj}_EP_PROPERTY_${property}} ${${_ALL_PROJECT_IDENTIFIER}_EP_PROPERTY_${property}})
-    if(_all)
+    list(LENGTH _all _num_properties)
+    if(_num_properties GREATER 0)
       list(APPEND _ep_arguments ${property} ${_all})
     endif()
   endforeach()