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/Cmdlets/GetMinIOObjectCmdlet.cs

@@ -108,7 +108,7 @@ namespace PSMinIO.Cmdlets
                 {
                     var wildcardPattern = new WildcardPattern(Name, WildcardOptions.IgnoreCase);
                     objects = objects.Where(o => o.Name != null && wildcardPattern.IsMatch(o.Name)).ToList();
-                    WriteVerboseMessage("Filtered to {0} objects matching pattern '{1}'", objects.Count, Name!);
+                    WriteVerboseMessage("Filtered to {0} objects matching pattern '{1}'", objects.Count, Name);
                 }
 
                 // Apply directory filters