You use NULL, as Scott mentioned.

create PROCEDURE [dbo].[spUpdateTableA]

@TableAPK int, --only required parameter

@ColumnA nvarchar(50) = NULL,

@ColumnB int = -1

as

begin

SET NOCOUNT ON

SET XACT_ABORT ON

declare @ErrorMessage nvarchar(4000);

begin try

--Start the transaction

begin transaction

update TableA

set ColumnA = CASE WHEN @ColumnA is null then ColumnA else @ColumnA end,

ColumnB=case when @ColumnB = -1 then ColumnB else @ColumnB end

where TableAPK=@TableAPK

COMMIT TRANSACTION