Electronic – Why does GET_REPORT on the HID Keyboards sometimes return an empty packet

No, D- is not ground. Data is sent over a differential line, which means that D- is a mirror image of D+, so both Data lines carry the signal. The receiver subtracts D- from D+. If some noise signal would be picked up by both wires, the subtraction will cancel it.

So differential signalling helps suppressing noise. So does the type of wiring, namely twisted pair. If the wires ran just parallel they would form a (narrow) loop which could pick up magnetic interference. But thanks to the twists the orientation of the wires with respect to the field changes continuously. An induced current will be cancelled by a current with the opposite sign half a twist further.
Suppose you have a disturbance working vertically on the twisted wire. You could regard each half twist as a small loop picking up the disturbance. Then it's easy to see that the next tiny loop sees the opposite field (upside down, so to speak), so that cancels the first field. This happens for each pair of half twists.
A similar balancing effect occurs for capacitance to ground. In a straight pair one conductor shows a higher capacitance to ground than the other, while in a twisted pair each wire will show the same capacitance.

edit
Cables with several twisted pairs like cat5 have a different twist length for each pair to minimize crosstalk.

If I recall correctly, the Physical Min and Max are just "window dressing" so to speak. From the HID spec:

While Logical Minimum and Logical Maximum (extents) bound the values returned by a device, Physical Minimum and Physical Maximum give meaning to those bounds by allowing the report value to be offset and scaled. For example, a thermometer might have logical extents of 0 and 999 but physical extents of 32 and 212 degrees.The resolution can be determined with the following algorithm:

So for this example, the HID thermometer would be returning a value between 0-999 in the report. That's the bytes that actually flow from device to PC through an IN endpoint. These values are determined by the Logical Min/Max.

The Physical Min/Max describes how the Logical units map onto a Physical "spectrum". In this case, from water's freezing (32 degrees F) to the boiling (212 F). In this case, when the device sends a 0, it means 32 F, and when it sends 999, it means 212 F. Software can then read the HID descriptor to determine the mapping.

In the grand scheme of things, physical min/max aren't that meaningful unless you intentionally use them. For consistency, I generally set my physical to match my logical.