fix: IsValid accepts empty strings for non-nullable string fields (20.0.3)

[erwan-joly]

Summary

Symptom from live NosCore login traffic:
```
EROR -- Sending an invalid packet: header=NsTeST type=NosCore.Packets.ServerPackets.Login.NsTestPacket errors=[The NsTestPacket field is required.]
```

PacketBase.IsValid constructs `new RequiredAttribute()` which defaults to `AllowEmptyStrings = false`. That rejects `NsTestPacket.LeadingBlank = ""` — even though an empty token is exactly what the wire carries.

Switch to `new RequiredAttribute { AllowEmptyStrings = true }` so a non-nullable string property can legally hold `""`.

Test plan

• `dotnet test` — 113/113 passing

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

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Run ID: 520fac3e-b772-4318-9d76-e807bbdb4da9

📥 Commits

Reviewing files that changed from the base of the PR and between 41cf996 and 662254e.

📒 Files selected for processing (2)

• src/NosCore.Packets/NosCore.Packets.csproj
• src/NosCore.Packets/PacketBase.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/allow-empty-strings-in-required-check

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