Is there such a thing as using an IF/ELSE in the WHERE clause
Absolutely, there is a CASE expression - not only in the WHERE clause, but also in other parts of your query. However, a more common approach is to use logical expressions in the WHERE clause that force SQL Server to take one condition or the other depending on the parameter setting.
my condition would be if @Customer = '' then select all
If you would like to select all customers when the parameter is set to empty, or select all customers where the parameter is not set, you can do this:

SELECT *
FROM SalesData
WHERE @Customer = ''
   OR  Customer = @Customer

If @Customer is set to '' , the first clause of the OR expression will be TRUE for all rows, so all customers would be returned. However, when @Customer is non-empty, the second part of the expression would be evaluated.

Not quite sure how you need to perform search
But can try something like below

SELECT *
FROM SalesData
WHERE Customer like '%' + @Customer + '%'

Or

SELECT *
FROM SalesData
WHERE (LEN(@Customer)>0 AND Customer =@Customer)
     OR (LEN(@Customer)=0)

In here I havent trim assuming you always pass '' . But you can always use LEN(LTRIM(RTRIM(@Customer)))

如果将@Customer默认为NULL而不是''，则只需

WHERE Customer = ISNULL(@Customer, Customer)