I inherited an application that uses some stored procedures and would like to get the groups opinion on the code. Below is a sample of one of the stored procedures used to insert or modify user data. The SQL Server version is Microsoft SQL Server 2014 - 12.0.4100.1 (X64).

USE [MyDataBase]
GO
/****** Object:  StoredProcedure [dbo].[InsertOrModifyUserData]    Script Date: 2/2/2016 8:39:57 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

ALTER  PROCEDURE [dbo].[InsertOrModifyUserData]
@userName NVARCHAR(200),
@isSuperAdmin BIT = NULL,
@modifiedBy NVARCHAR(200),
@modifiedDate DATETIME = NULL,
@isActive BIT = NULL
AS

BEGIN
    SET NOCOUNT ON;    
    SET XACT_ABORT ON; 
    BEGIN TRY
        BEGIN TRANSACTION

    
            IF (@modifiedDate IS NULL OR @modifiedDate ='1900-01-01 00:00:00.000')
                SET @modifiedDate = GETDATE()
            IF (@isActive IS NULL)
                SET @isActive = 1


            IF ((@userName IS NOT NULL OR @userName <>'') AND (@isSuperAdmin IS NOT NULL OR @isSuperAdmin<>'') AND (@modifiedBy IS NOT NULL OR @modifiedBy <>''))
            BEGIN 
                IF EXISTS(SELECT UserName FROM [dbo].[UserAccess] WHERE UserName = @userName)
                    UPDATE  [dbo].[UserAccess]  
                        SET IsSuperAdmin    = @isSuperAdmin,
                            ModifiedBy      = @modifiedBy,
                            ModifiedDate    =   @modifiedDate,
                            IsActive        = @isActive 
                    WHERE   UserName = @userName    
                ELSE
                    INSERT INTO [dbo].[UserAccess] (UserName, IsSuperAdmin, CreatedDate, ModifiedBy, ModifiedDate,IsActive)
                    VALUES(@userName, @isSuperAdmin, GETDATE(), @modifiedBy, @modifiedDate, @isActive)  
        
            END 
        ELSE
            RAISERROR('Required parameters are not provided or Required parameters are passed as NULL',13,1)
        COMMIT TRANSACTION
        select null
    END TRY
    BEGIN CATCH
            DECLARE @ErrorMessage NVARCHAR(MAX) = ERROR_MESSAGE();
            DECLARE @ErrorSeverity INT = ERROR_SEVERITY();
            DECLARE @ErrorState INT = ERROR_STATE();
            RAISERROR(@ErrorMessage, @ErrorSeverity, @ErrorState);
            ROLLBACK TRANSACTION;
    END CATCH;

END