refactor: remove redundant null checks

SongTidy.Tests/RenameFormatsTests.cs

@@ -114,15 +114,6 @@ public sealed class RenameFormatsTests {
         AssertStem(RenameFormat.ArtistFirst, "   ", "Title", "\t", "Title");
     }
 
-    /// <summary>
-    /// Verifies that a null title is rejected.
-    /// </summary>
-    [TestMethod]
-    public void ThrowsForNullTitle() {
-        Assert.Throws<ArgumentNullException>(
-            () => RenameFormats.GenerateStem(RenameFormat.ArtistFirst, "Artist", null!, "Live"));
-    }
-
     #endregion
 
     /// <summary>