fix(core): improve null safety and default values in PepHelper

- Add null-conditional operator for dwg.QtyRequired (dwg?.QtyRequired ?? 0)
- Use string.Empty instead of null for plate names
- Remove unused using statements

These changes prevent potential null reference exceptions and provide
better default values.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

PepApi.Core/PepHelper.cs

@@ -1,5 +1,3 @@
-using System.Collections.Generic;
-using System.Linq;
 using PepLib;
 using Material = PepApi.Core.Models.Material;
 using Part = PepApi.Core.Models.Part;
@@ -171,7 +169,7 @@ namespace PepApi.Core
                 var part = new Part
                 {
                     Name = drawing.Name,
-                    QtyRequired = dwg.QtyRequired
+                    QtyRequired = dwg?.QtyRequired ?? 0
                 };
 
                 UpdateQtyNested(part, nest);
@@ -217,7 +215,7 @@ namespace PepApi.Core
             foreach (var item in uniquePlates)
             {
                 var plate = item.First();
-                plate.Name = null;
+                plate.Name = string.Empty;
                 plate.Qty = item.Sum((Plate p) => p.Qty);
 
                 combinedPlatesList.Add(plate);