Record in ssrs report when parameter is null

Hi

I crated a report that has a parameter. I want to show all records when parameter is null

for example below is my query

<fetch distinct="false" no-lock="false" mapping="logical"><entity name="account" enableprefiltering="1" prefilterparametername="CRM_FilteredAccount"><attribute name="name" alias="name" /><attribute name="accountid" /><filter type="and"><condition attribute="name" value="@a" operator="eq"/></filter></entity></fetch>

if parameter "@a" is blank or null it shows no record

I want that if "@a" is blank or null it skip that condition and show all Accounts

Thanks in Advance