`NonprofitMailer.refund` incorrectly tries to send even if nothing in `to:`

It's possible for the nonprofit to have no users set to receive notifications of payments, i.e. the call on line 23

Lines 23 to 24 in 095d28b

    
           @emails = QueryUsers.nonprofit_user_emails(@nonprofit.id, 'notify_payments') 
        
           mail(to: @emails, subject: "A new refund has been made for $#{Format::Currency.cents_to_dollars(@refund.amount)}")

returns an empty array. That makes the call to mail on line 24 error out (at least if you're using AWS SES) as it's incorrect to send an email with nothing in the "to:" header.

The fix would be to add a guard on line 24 against @emails being an empty array.

Me (@victordscabral), @kayrocesar and @joaobisi will work on it.

Link to the PR: #1460

	@emails = QueryUsers.nonprofit_user_emails(@nonprofit.id, 'notify_payments')
	mail(to: @emails, subject: "A new refund has been made for $#{Format::Currency.cents_to_dollars(@refund.amount)}")