It is not wise to check the end of an array by looking at its content.
A "0" is otherwise used as invalid array entry.
Replaced this usage in nvidiagpu.pm and amgpu.pm by comparing indices with the size directly.
- Devices are no independent identigiers in a list.
- Sensors specify the absolute path to the file containing the information.
- Unavailable sensors have to be left blank.
- Some indentations fixes.