fix: IsValid accepts empty strings for non-nullable string fields (20.0.3) (#461)

PacketBase.IsValid constructed RequiredAttribute with default
AllowEmptyStrings=false, so a non-nullable string property with an
empty value (e.g. NsTestPacket.LeadingBlank = "" that models the
literal double-space token) failed validation with
"The <Type> field is required.". Empty tokens are legal on the wire;
allow them.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erwan-joly

src/NosCore.Packets/NosCore.Packets.csproj

@@ -12,7 +12,7 @@
 		<RepositoryUrl>https://github.com/NosCoreIO/NosCore.Packets.git</RepositoryUrl>
 		<PackageIconUrl></PackageIconUrl>
 		<PackageTags>nostale, noscore, chickenapi, nostale private server source, nostale emulator</PackageTags>
-		<Version>20.0.2</Version>
+		<Version>20.0.3</Version>
 		<PackageRequireLicenseAcceptance>false</PackageRequireLicenseAcceptance>
 		<Description>NosCore's Packets (Nostale packets) defined over classes</Description>
 		<PackageLicenseExpression>MIT</PackageLicenseExpression>

src/NosCore.Packets/PacketBase.cs

@@ -52,7 +52,7 @@ public bool IsValid
                     var nullabilityInfo = nullabilityContext.Create(prop);
                     if (nullabilityInfo.WriteState is not NullabilityState.Nullable)
                     {
-                        var attr = new RequiredAttribute();
+                        var attr = new RequiredAttribute { AllowEmptyStrings = true };
                         var result = attr.GetValidationResult(value, vc);
                         if (result != null)
                         {