Fix calibration defaulting

[ToddSchumannNTIA]

Fixes an issue where the sensor calibration values would not properly default to the sigan values when using an empty or sparse sensor calibration file.

• Added fix
• Added additional pytest for this situation

Example empty calibration matrix:

            10e6: {  # Sample rate
                1e9: {  # Center frequency
                    40: {},  # Gain setting and empty calibration dict
                    60: {}
                },
                2e9: {
                    40: {},
                    60: {}
                }
            }
        }

Original issue description:
When using an empty calibration file or a file where not all values are specified, scos-sensor will correctly load the file, which prevented reloading the values to be overwritten into the currect calibration dict (set in recompute_calibration_data in usrp_radio.py). When retuned, the values from the previous tuning were never overwritten from the defaulting values, so new default values were not used.