How to make conditional formatting exclude blank cells

Making a working hours chart but I'm not very well versed in many Excel functions so I've been consulting google results for most, but I don't understand why conditional formatting is marking blank cells green for value >0. The detailed issue is as follows:

I have columns for for date (C), start time (D), end time (E), and the following (using row 5 for example):

Total time F5 =IF(D5;E5-D5;"") Number format: Time

Overtime: G5 =IF(OR(F40="";TIME(8;;)="");"";(F40-TIME(8;;))*24) Number format: General

I have set up conditional formatting in column G to color everything that is below 0 red, everything above 0 green, but all the blank cells with above formula applied are colored green too. Cells that equal 0 are left without color, as I intended.

My hope is that blank cells also remain white until they recieve a value. Is there a way to achieve that?

I appreciate all the help!