Blank function/Null function for missing dates

Hello I have an excel spreadsheet that has dates of when a project report was originally sent to the client (D2, D3, etc.,) and I have dates of when we finally received payment for the report (M2, M3, etc.,). As well I have a column that counts the days between these two time frames (N2, N3) so we can figure out how long it takes to receive payment. (=M2-D2, =M3-D3, etc.,).

My problem is that when there isn't a date in the payment received column (M3 in the spreadsheet below), that it brings over the date from D3 in a number format into the Total Number of Days to Receive Payment column (N3) and doesn't look right. How can i create a function to automatically update N3 (Total # of days column) when there's a number in M3 but to stay blank if M3 is blank.

Take a look at my spreadsheet and see if this makes sense Thank you! you are GREATLY appreciated. I work and work and work on these till I feel like I want to pull my hair out, and then I give it to the masters to show me how it's done.