fix: do not append tail if it is null

Signed-off-by: leo <longshuang@msn.cn>

Author: love-linger

src/Models/DiffResult.cs

@@ -254,7 +254,8 @@ public void GeneratePatchFromSelection(Change change, string fileTreeGuid, TextD
                 }
             }
 
-            writer.WriteLine($" {tail}");
+            if (!string.IsNullOrEmpty(tail))
+                writer.WriteLine($" {tail}");
             writer.Flush();
         }
 
@@ -405,7 +406,8 @@ public void GeneratePatchFromSelectionSingleSide(Change change, string fileTreeG
                 }
             }
 
-            writer.WriteLine($" {tail}");
+            if (!string.IsNullOrEmpty(tail))
+                writer.WriteLine($" {tail}");
             writer.Flush();
         }