Remove uses of .NET 4+ only API

com.unity.shadergraph/Editor/Drawing/Blackboard/BlackboardProvider.cs

             List<string> newStrings = new List<string>();
             foreach (string s in splitString)
             {
-                if (!string.IsNullOrWhiteSpace(s))
+                var str = s.Trim();
+                if (!string.IsNullOrEmpty(str))
-                    newStrings.Add(s.Trim());
+                    newStrings.Add(str);
                 }
             }
 

com.unity.shadergraph/Editor/Drawing/SearchWindowProvider.cs

                 foreach (var guid in AssetDatabase.FindAssets(string.Format("t:{0}", typeof(MaterialSubGraphAsset))))
                 {
                     var asset = AssetDatabase.LoadAssetAtPath<MaterialSubGraphAsset>(AssetDatabase.GUIDToAssetPath(guid));
-                    var title = asset.subGraph.path.Split('/').Append(asset.name).ToArray();
+                    var title = asset.subGraph.path.Split('/').ToList();
+                    title.Add(asset.name);
-                    AddEntries(node, title, nodeEntries);
+                    AddEntries(node, title.ToArray(), nodeEntries);
                 }
             }