fix: prevent null startLine from causing unnecessary line append in CodeBuilder

src/Lua.SourceGenerator/Utilities/CodeBuilder.cs

@@ -11,7 +11,7 @@ internal sealed class CodeBuilder
         public IndentScope(CodeBuilder source, string? startLine = null)
         {
             this.source = source;
-            source.AppendLine(startLine);
+            if(startLine!=null) source.AppendLine(startLine);
             source.IncreaseIndent();
         }
 
@@ -28,7 +28,7 @@ internal sealed class CodeBuilder
         public BlockScope(CodeBuilder source, string? startLine = null)
         {
             this.source = source;
-            source.AppendLine(startLine);
+            if(startLine!=null) source.AppendLine(startLine);
             source.BeginBlock();
         }