I am working on a visualforce page that allows a user to uppdate an opportunity and opportunityLineItems and give them the option of attaching a file. The problem I am running into is that if you don't attach a file I get the error below. The user needs to be able to add or not add the attachment. Any suggestions?

This is the error:
System.DmlException: Insert failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Required fields are missing: [File Name, Body]: [File Name, Body]
Error is in expression '{!saveIt}' in component <apex:commandButton> in page orderonhand: Class.OrderOnHandClass.saveIt: line 30, column 1
Class.OrderOnHandClass.saveIt: line 30, column 1

This is the Visualforce Page:

<apex:page standardController="Opportunity" extensions="OrderOnHandClass">
    <apex:form >
        <apex:pageBlock >
        
            <apex:pageBlockSection title="Opportunity Details">
                <apex:inputCheckbox value="{!opportunity.Order_on_Hand__c}"/>
                <apex:inputField value="{!Opportunity.PO_Number__c}"/>
                <apex:inputField value="{!Opportunity.Order_Ship_To_Location__c}"/>
                
            </apex:pageBlockSection> 
            
            <apex:pageBlockSection title="Opportunity Product Details">
                <apex:pageBlockTable var="OLI" value="{!OLIs}" id="newProduct">
                    <apex:column value="{!OLI.ProductCode}"/>
                    <apex:column headerValue="Quantity">        
                        <apex:inputfield value="{!OLI.Quantity}"/>        
                    </apex:column>
                    <apex:column headerValue="Order Quantity if Different">        
                        <apex:inputfield value="{!OLI.Order_Quantity_if_Different__c}"/>        
                    </apex:column>
                    <apex:column headerValue="Approved Price">        
                        <apex:outputfield value="{!OLI.Approved_Price__c}"/>        
                    </apex:column>
                </apex:pageBlockTable>
            </apex:pageBlockSection> 
            
            <apex:pageBlockButtons >
                <apex:commandButton value="Save" action="{!saveIt}"/>
                <apex:commandButton value="Cancel" action="{!cancel}"/>
            </apex:pageBlockButtons>
           
            <apex:pageBlockSection title="Attachment" >
            Attach PO: <apex:inputfile value="{!a.body}" filename="{!a.name}"></apex:inputfile>
            </apex:pageBlockSection>

        </apex:pageBlock>
    </apex:form>
</apex:page>

This is the Class:

public class OrderOnHandClass {

    public ApexPages.StandardController sc;
    public Opportunity Opp {get;set;}
    public List<OpportunityLineItem> OLIlist2 {get ;set;}
    public Attachment a {get;set;}
      
    public OrderOnHandClass(ApexPages.StandardController sc) { 
        this.Opp = (Opportunity)sc.getRecord();
        OLIlist2 = [Select Name, ID,ProductCode, Quantity,Approved_Price__c, OpportunityId,Order_Quantity_if_Different__c FROM OpportunityLineItem WHERE OpportunityId =:Opp.Id];
        this.a = new Attachment();
       
        }
            
    public List<OpportunityLineItem> getOLIs() {
        return OLIlist2;
    }

    public PageReference saveIt() {
        
            opp.stageName = 'Order Received';
            update Opp;
             
            update OLIlist2;
       
       
        if(a.Id == Null){
            a.parentId = opp.Id;
        insert a;
        }else{
            Return Null;
            }
            
        String p = ApexPages.currentPage().getParameters().get('param');
            if(p == null){
                PageReference pageRef = new PageReference('https://cs41.salesforce.com/'+opp.Id);
                return pageRef;
            }
            else{
                return null;
            }     
    }
    }