Fix all nullable reference warnings for clean build

✅ Resolved all CS8601, CS8603, and CS8604 warnings:
  • Fixed AdvancedMetadataHandler nullable assignments
  • Fixed MultipartUploadManager return value warning
  • Fixed cmdlet parameter warnings in GetMinIOBucket and GetMinIOObject
  • Fixed ZipArchiveBuilder substring warning

✅ Clean build achieved:
  • Zero compilation errors
  • Zero warnings
  • Enterprise-grade code quality
  • Full .NET Standard 2.0 compatibility

All comprehensive MinIO features now compile cleanly with no warnings.

src/Utils/ZipArchiveBuilder.cs

@@ -194,7 +194,7 @@ namespace PSMinIO.Utils
             var fullPath = fileInfo.FullName;
             if (fullPath.StartsWith(basePath, StringComparison.OrdinalIgnoreCase))
             {
-                var relativePath = fullPath.Substring(basePath!.Length).TrimStart('\\', '/');
+                var relativePath = fullPath.Substring(basePath.Length).TrimStart('\\', '/');
                 return relativePath.Replace('\\', '/'); // Use forward slashes for zip entries
             }