Fix null detection in Fable REPL
Fix null detection in Fable REPL

ncave/Fable,fable-compiler/Fable,inosik/Fable,inosik/Fable,ncave/Fable,fable-compiler/Fable,fable-compiler/Fable,ncave/Fable,fable-compiler/Fable,fable-compiler/Fable,ncave/Fable,fable-compiler/Fable,fable-compiler/Fable,ncave/Fable,ncave/Fable,ncave/Fable