refactor: remove redundant null checks

SongTidy.Tests/FilenameParserTests.cs

@@ -231,18 +231,6 @@ public sealed class FilenameParserTests {
 
     #endregion
 
-    #region Argument validation
-
-    /// <summary>
-    /// Verifies that a null input is rejected.
-    /// </summary>
-    [TestMethod]
-    public void ThrowsForNullInput() {
-        Assert.Throws<ArgumentNullException>(() => FilenameParser.Parse(null!));
-    }
-
-    #endregion
-
     /// <summary>
     /// Parses the input and asserts all three resulting fields.
     /// </summary>