Need help with conditional formatting using an IF function to recognise blank cells and overdue dates

Hi

I am wanting to add conditional formatting to a spreadsheet which lists invoices and their due dates, along with payment date.

One column (H) has the payment due date.

Another column (M) has the date paid. This column is left blank if unpaid to date.

I want to highlight only cells in column H in which the due date is anything less than today, but ignoring those where there is a value in column M already. Essentially, I do not want to highlight anything which is overdue, but already paid (if that makes sense).

Having trouble working out what formula I need to do to show this in the conditional formatting area. I can get it to do the dates past easy enough, but cant get it to also ignore blank cells.

Have tried using ISBLANK.

Would greatly appreciate anyone's help with this formula!!

Thanks!!