When selecting % from one of my report parameters, null fields are not returned

An example of one of the datasets for the report parameter:

SELECT Display AS PlansAdmin
FROM LUGenInfoPlansAdmin
WHERE Display IS NOT NULL
UNION
SELECT '%' AS PlansAdmin
ORDER BY PlansAdmin

The WHERE clause of my result set
WHERE (Projects.WMPlansAdmin LIKE @PlansAdmin)

My question is, can I dynamically change the where clause to include NULLS
(Projects.WMPlansAdmin LIKE @PlansAdmin OR Projects.WMPlansAdmin IS NULL)
when users select the '%' parameter?

I've tried setting the where clause as a variable, but this gives me a syntax error:
DECLARE @WhereClause VARCHAR(100)
  IF (@Display = '%')
     SET @WhereClause = '@Projects.WMPlansAdmin LIKE ' + @Display + ' OR Projects.WMPlansAdmin IS NULL'
  ELSE
     SET @WhereClause = '@Projects.WMPlansAdmin LIKE ' + @Display

SELECT ...
FROM ...
WHERE @WhereClause


..any suggestions?

Any luck with this? I have the same issue.

I know I can do this but I want to have multiple parameters.

IF @code is null
  BEGIN
    'sql without the where for the cust code
  END
ELSE
  BEGIN
    'sql with the where for the cust code
  END

This solves the problem:

WHERE (isnull(wbw_line.CUST_CODE,'') LIKE @code)