Genex: Refactor empty element strip

In cmGeneratorExpressionEvaluator, teach getLinkedTargetsContent to call
cmGeneratorExpression::StripEmptyListElements to transform its return
value so that callers do not have to do so.

Source/cmGeneratorExpressionEvaluator.cxx

@@ -841,6 +841,8 @@ getLinkedTargetsContent(
       context->HadContextSensitiveCondition = true;
       }
     }
+  linkedTargetsContent =
+    cmGeneratorExpression::StripEmptyListElements(linkedTargetsContent);
   return linkedTargetsContent;
 }
 
@@ -1100,9 +1102,6 @@ static const struct TargetPropertyNode : public cmGeneratorExpressionNode
         }
       }
 
-    linkedTargetsContent =
-          cmGeneratorExpression::StripEmptyListElements(linkedTargetsContent);
-
     if (!prop)
       {
       if (target->IsImported()