Pipes refactoring, Fixes in type checker

Src/Data/Myc.Data.Keyword.pas

@@ -86,12 +86,14 @@ type
         class var
             FLock: TSpinLock;
             FRegistry: TDictionary<TArray<Integer>, IKeywordMapping<T>>;
+            FEmpty: IKeywordMapping<T>;
         class constructor Create;
         class destructor Destroy;
 
     public
         // Creates or gets the cached keyword mapping.
         class function Intern(const AFields: TArray<TPair<IKeyword, T>>): IKeywordMapping<T>; static;
+        class property Empty: IKeywordMapping<T> read FEmpty;
     end;
 
     TGenericRecord<T> = class(TInterfacedObject, IKeywordMapping<T>)
@@ -270,6 +272,8 @@ class constructor TKeywordMappingRegistry<T>.Create;
 begin
     FRegistry := TDictionary<TArray<Integer>, IKeywordMapping<T>>.Create(TEqualityComparer<TArray<Integer>>.Default);
     FLock := TSpinLock.Create(False);
+
+    FEmpty := Intern([]);
 end;
 
 class destructor TKeywordMappingRegistry<T>.Destroy;