test: Ensure handling of null arguments

AzureLiquid.Preview/PreviewProcess.cs

@@ -162,7 +162,7 @@ private static void LogMissingFiles(PreviewProcess preview)
     /// <param name="preview">The instance of <see cref="PreviewProcess" /> to handle the output.</param>
     private static void HandleNoArgumentsPassed(string[] args, PreviewProcess preview)
     {
-        if (args.Length == 0)
+        if (args == null || args.Length == 0)
         {
             preview.WriteHelpOutput();
         }

AzureLiquid.Preview/PreviewProcessArguments.cs

@@ -53,7 +53,7 @@ internal static bool IsArgMatch(string arg, string key)
     /// <param name="args">The passed arguments.</param>
     /// <param name="key">The key.</param>
     /// <returns><c>true</c> if the argument was found, otherwise <c>false</c>.</returns>
-    public static bool HasArgument(string[] args, string key) => args.Any(arg => IsArgMatch(arg, key));
+    public static bool HasArgument(string[] args, string key) => args?.Length > 0 && args.Any(arg => IsArgMatch(arg, key));
 
     /// <summary>
     /// Parses the argument value.
@@ -63,6 +63,11 @@ internal static bool IsArgMatch(string arg, string key)
     /// <returns>The argument value.</returns>
     public string ParsePath(string[] args, string key)
     {
+        if (args == null || args.Length == 0)
+        {
+            return string.Empty;
+        }
+
         var index = GetArgumentIndex(args, key);
         return
             index == -1 || index - 1 >= args.Length

AzureLiquid.Tests/PreviewProcessTests.cs

@@ -161,6 +161,23 @@ public void EnsureFileReadExceptionHandling()
         instance.Log.Should().NotBeEmpty("A log should not have been created");
     }
 
+    /// <summary>
+    /// Ensure the preview process can handle missing arguments.
+    /// </summary>
+    [Fact]
+    public void EnsureHelpMessageShown()
+    {
+        // Arrange
+        var instance = PreviewProcess.Create(null!);
+
+        // Act
+        var result = instance.Render();
+
+        // Assert
+        result.Should().BeEmpty("A result should not have been created");
+        instance.Log.Should().NotBeEmpty("A log should not have been created");
+    }
+
     /// <summary>
     /// Contains arranged values used for testing, containing mock instances and expected return values.
     /// </summary>