Sync PathHelpers.SafePathCombine null validation with TemplateDotNetTool

src/DemaConsulting.VersionMark/PathHelpers.cs

@@ -34,6 +34,10 @@ internal static class PathHelpers
     /// <exception cref="ArgumentException">Thrown when relativePath contains invalid characters or path traversal sequences.</exception>
     internal static string SafePathCombine(string basePath, string relativePath)
     {
+        // Validate inputs
+        ArgumentNullException.ThrowIfNull(basePath);
+        ArgumentNullException.ThrowIfNull(relativePath);
+
         // Ensure the relative path doesn't contain path traversal sequences
         if (relativePath.Contains("..") || Path.IsPathRooted(relativePath))
         {

test/DemaConsulting.VersionMark.Tests/PathHelpersTests.cs

@@ -26,6 +26,36 @@ namespace DemaConsulting.VersionMark.Tests;
 [TestClass]
 public class PathHelpersTests
 {
+    /// <summary>
+    ///     Test that SafePathCombine throws ArgumentNullException when basePath is null.
+    /// </summary>
+    [TestMethod]
+    public void PathHelpers_SafePathCombine_NullBasePath_ThrowsArgumentNullException()
+    {
+        // Arrange
+        string? basePath = null;
+        var relativePath = "subfolder/file.txt";
+
+        // Act & Assert
+        Assert.Throws<ArgumentNullException>(() =>
+            PathHelpers.SafePathCombine(basePath!, relativePath));
+    }
+
+    /// <summary>
+    ///     Test that SafePathCombine throws ArgumentNullException when relativePath is null.
+    /// </summary>
+    [TestMethod]
+    public void PathHelpers_SafePathCombine_NullRelativePath_ThrowsArgumentNullException()
+    {
+        // Arrange
+        var basePath = "/home/user/project";
+        string? relativePath = null;
+
+        // Act & Assert
+        Assert.Throws<ArgumentNullException>(() =>
+            PathHelpers.SafePathCombine(basePath, relativePath!));
+    }
+
     /// <summary>
     ///     Test that SafePathCombine correctly combines valid paths.
     /// </summary>