Google Sheets – How to Add Condition to Formula to Not Trigger if Another Cell is Blank

I'm new to adding formulas etc. in Google Sheets, and I'm currently trying to set up a library borrowing/returns spreadsheet.

I have three columns formatted to dates: 'G' (date borrowed), 'H' (date due) and 'I' (date returned).

In 'H' I have the following formula to count 20 days from the date in column 'G', whilst also not firing if 'G' is blank:

=ArrayFormula(if(ISBLANK(G6),"",DATEVALUE(G6)+20))

This works great.

I also have conditional format to highlight 'H' in red if the date becomes overdue, which is:

Value is less than
=TODAY()

This does the job initially, except that I need the format which makes 'H' highlight red NOT to fire if there is a return date entered in column 'I' (otherwise every entry will eventually turn red, and that's not really helpful for what I'm doing!).

I'm having a hard time figuring it out.

I feel it should be simple to add an ISBLANK type command into the =TODAY() format, but I'm clearly missing something.

Any help would be greatly appreciated.