Check if the path is valid when using WithPath(...) (#1377)

src/WireMock.Net.Abstractions/Validators/PathValidator.cs

@@ -0,0 +1,19 @@
+// Copyright © WireMock.Net
+
+using System;
+
+namespace WireMock.Validators;
+
+public static class PathValidator
+{
+    /// <summary>
+    /// A valid path must start with a '/' and cannot be null, empty or whitespace.
+    /// </summary>
+    public static void ValidateAndThrow(string? path, string? paramName = null)
+    {
+        if (string.IsNullOrWhiteSpace(path) || path?.StartsWith("/") == false)
+        {
+            throw new ArgumentException("Path must start with a '/' and cannot be null, empty or whitespace.", paramName ?? nameof(path));
+        }
+    }
+}