ExternalProject: error out only if the property is unset

Property values may be empty; instead make sure they exist and allow for
empty values.

Modules/ExternalProject.cmake

@@ -1184,10 +1184,11 @@ endfunction()
 function(ExternalProject_Get_Property name)
   foreach(var ${ARGN})
     string(TOUPPER "${var}" VAR)
-    get_property(${var} TARGET ${name} PROPERTY _EP_${VAR})
-    if(NOT ${var})
+    get_property(is_set TARGET ${name} PROPERTY _EP_${VAR} SET)
+    if(NOT is_set)
       message(FATAL_ERROR "External project \"${name}\" has no ${var}")
     endif()
+    get_property(${var} TARGET ${name} PROPERTY _EP_${VAR})
     set(${var} "${${var}}" PARENT_SCOPE)
   endforeach()
 endfunction()