The "Null" is coming from invoice/quote general notes.
The invoice/quote notes section comprises two elements.
One is general notes, which will appear on every single quote and is where most of the users place terms and conditions. If you'd like to edit the general notes or remove the "Null", you can only do so through the web. This will apply to all quote/invoice printouts:
Step 1: On the side menu, click Financial
Step 2: Click Invoice, Statement & Reporting
Step 3: On the pop-ups, edit the general note for Quote and Invoices, then click Save
The other one is a note for an individual quote/invoice: the information you have put down under section "Notes" in the edit quote/invoice screen. They're the information tailored to this quote/invoice. Once the "Null" in general notes has been removed, they won't appear here.