Change -1 argument to None, change import name to avoid warning

[colbeyhair]

When calling the read_raw argument of the visa resource, the default argument of -1 yields the following error:
ValueError: Array length must be >= 0, not -1

Changing the argument to 'None' removes this error.

This argument is ultimately passed to the _read_raw function in pyvisa, for which the default argument is None. Source