fix saveLines appendLines so that empty and trailing newline is correct

src/main/scala/introprog/IO.scala

@@ -51,7 +51,7 @@ object IO:
     * @param enc the encoding of the file.
     * */
   def saveLines(lines: Seq[String], fileName: String, enc: String = "UTF-8"): Unit =
-    saveString(lines.mkString("\n") + "\n", fileName, enc)
+    if lines.nonEmpty then saveString(lines.mkString("", "\n", "\n"), fileName, enc)
 
   /**
     * Appends `string` to the text file `fileName` using encoding `enc`.
@@ -78,7 +78,7 @@ object IO:
     * @param enc the encoding of the file.
     * */
   def appendLines(lines: Seq[String], fileName: String, enc: String = "UTF-8"): Unit =
-    appendString(lines.mkString("\n"), fileName, enc)
+    if lines.nonEmpty then appendString(lines.mkString("","\n","\n"), fileName, enc)
 
   /**
     * Load a serialized object from a binary file called `fileName`.