You need to parenthesize your WHERE expression correctly:

WHERE (pp.project_leader =Project_Leader
        OR Project_Leader is null)
and (pp.rd_plan =RD_Plan
        OR RD_Plan is null)
and (pp.approval_status = Approval_Status
        OR Approval_Status is null)
and (pp.design_plan = Design_Plan
        OR Design_Plan is null)
and PP.actual_completion is null;

because AND has higher precedence than OR.

You need to parenthesize your WHERE expression correctly:

WHERE (pp.project_leader = ps_Project_Leader
        OR ps_Project_Leader is null)
and (pp.rd_plan = ps_RD_Plan
        OR ps_RD_Plan is null)
and (pp.approval_status = ps_Approval_Status
        OR ps_Approval_Status is null)
and (pp.design_plan = ps_Design_Plan
        OR ps_Design_Plan is null)
and PP.actual_completion is null;

because AND has higher precedence than OR.