Contact summary trigger w/test

SummarizeAccountContacts

trigger SummarizeAccountContacts on Contact (after delete, after insert) {

    set<ID> accountIds = new set<ID>();
    set<ID> contactIds = new Set<ID>();
   
    public void addValidAccountId(Id i)
    {
        if(i !=null)
            accountIds.add(i);
    }
    
    if(trigger.isInsert){  
        for(Contact contact : trigger.new){
            addValidAccountId(contact.AccountId);
        }
    }

    if(trigger.isDelete){
        for(Contact contact : trigger.old){
            addValidAccountId(contact.AccountId);
        }
    }

    //Validate existence of accounts to update
    if(accountIds == Null)
        return; //This is the only untested line, I'm not sure how to trigger a trigger-failure
    
    List<AggregateResult> contactsByAccount = [
        select 	AccountId, 
        count(ID) numContacts 
        from Contact 
        where AccountId in : accountIds Group By AccountId
    ];
    
    Map<Id,Integer> contactCountMap = new Map<Id,Integer>();
    
    //Convert list results to a map
    for (AggregateResult contactCount : contactsByAccount) {
        contactCountMap.put(
            string.valueOf(contactCount.get('AccountId'))
            , Integer.valueOf(contactCount.get('numContacts'))
        );
    } 
    
    List<Account> updateAccounts = new  List<Account>();
    
    //Create temporary accounts to update Num_Contacts__c
    for(Id accountId : accountIds ){
        Account tmpAccount = new Account(id=accountId);
        tmpAccount.Num_Contacts__c = contactCountMap.get(accountId);  
        updateAccounts.add(tmpAccount);
    } 
    
    update updateAccounts;    
}

TestSummarizeAccountContacts

@isTest
public class TestSummarizeAccountContacts {
    public static testMethod void validateContactCountOnAddition() {
        Account accountPlaceHolder = new Account();
        Account tmpAccount = new Account(
			Name='Acme'
		);
        
        insert tmpAccount;
        
        System.assertEquals(tmpAccount.Num_Contacts__c, null);
        
        Contact tmpContact = new Contact(
            LastName='Evans',
            FirstName='Bob',
            AccountID= tmpAccount.ID
		);
        
        insert tmpContact;
        
        accountPlaceHolder = [
            SELECT 
            	ID
            	, Num_Contacts__c 
            FROM Account 
            WHERE ID = :tmpAccount.id
        ];
        
        System.assertEquals(accountPlaceHolder.Num_Contacts__c,1);
    }
    
    public static testMethod void validateContactCountOnSubtraction() {
        Account accountPlaceHolder1 = new Account();
        Account tmpAccount1 = new Account(
			Name='Wyane Industries'
		);
        
        insert tmpAccount1;
        
        System.assertEquals(tmpAccount1.Num_Contacts__c, null);
        
        Contact tmpContact1 = new Contact(
            LastName='Evans',
            FirstName='Bob',
            AccountID= tmpAccount1.ID
		);
        
        Contact tmpContact2 = new Contact(
            LastName='Wyane',
            FirstName='Bruce',
            AccountID= tmpAccount1.ID
		);
        
        System.assertEquals(tmpAccount1.Num_Contacts__c, null);
        
        insert tmpContact1;
        insert tmpContact2;
        
        accountPlaceHolder1 = [
            SELECT 
            	ID
            	, Num_Contacts__c 
            FROM Account 
            WHERE ID = :tmpAccount1.id
        ];
        
        System.assertEquals(accountPlaceHolder1.Num_Contacts__c,2);
        
        delete tmpContact2;
        
        accountPlaceHolder1 = [
            SELECT 
            	ID
            	, Num_Contacts__c 
            FROM Account 
            WHERE ID = :tmpAccount1.id
        ];
        
        System.assertEquals(accountPlaceHolder1.Num_Contacts__c,1);
    }
    
    public static testMethod void validateContactCountNonRelatedAccount() {
        Account accountPlaceHolder = new Account();
        Account tmpAccount = new Account(
			Name='Acme'
		);
        
        Account tmpAccount2 = new Account(
			Name='Wyane Industries'
		);
        
        insert tmpAccount;
        
        System.assertEquals(tmpAccount.Num_Contacts__c, null);
        System.assertEquals(tmpAccount2.Num_Contacts__c, null);
        
        Contact tmpContact = new Contact(
            LastName='Evans',
            FirstName='Bob',
            AccountID= tmpAccount.ID
		);
        
        insert tmpContact;
        
        accountPlaceHolder = [
            SELECT 
            	ID
            	, Num_Contacts__c 
            FROM Account 
            WHERE ID = :tmpAccount.id
        ];
        
        System.assertEquals(accountPlaceHolder.Num_Contacts__c,1);
        
        //Not exactly sure how to trigger a trigger-failure, this is my best guess
        System.assertEquals(tmpAccount2.Num_Contacts__c,null);
    }
    
}