LIKE and NULL in WHERE clause in SQL

LIKE and NULL in WHERE clause in SQL

I have a store procedure which i have planned to use for search and get
all values.
Scenario: If the parameter passed is NULL it should return all the values
of the table and if the parameter passed is not NULL it should return the
values according to the condition which is in LIKE.
//Query:
ALTER procedure [dbo].[usp_GetAllCustomerDetails]
(
@Keyword nvarchar(20) = null
)
As
Begin
Select
CustomerId,CustomerName,CustomerTypeName,CustomerCode,CategoryName,CustomerMobile,CustomerEmail,CustomerAddress,CustomerCity,CustomerState,Pincode
from tblCustomerMaster CM
inner join dbo.tblCustomerTypeMaster CTM on CTM.CustomerTypeId =
CM.CustomerType
inner join dbo.tblCategoryMaster CCM on CCM.CategoryId= CM.CustomerCategory
where CustomerName like '%'+@Keyword+'%'
In the above query it returns no values when i execute since the NULL is
assumed as string by SQL, so what should i write in the where clause to
get the desired output?