help with a parameter in a query

 helloi have parameter in a SQL query 

i need to retrieve the results whereas the parameters hold a value

but if the parameter doesn't hold any value

 i want to retrieve all results i want to omit the statement of the parameter

 Notice that my relevant statement in the blue bold line

here is my query

select 

               initcap(x_header.EVENT_TYPE_CODE) type_indicator, 

                -- 'Apply/u PrePayement' type_indicator,  

               to_char(inv.invoice_num) invoice_or_check_number,

               x_header.ACCOUNTING_DATE AS_GL_date,    

               x_lines.UNROUNDED_ACCOUNTED_DR amount_dr_usd,

               x_lines.UNROUNDED_ACCOUNTED_CR amount_cr_usd,