Handle empty 'keys' attributes
The 'key' sysfs attribte is present for a device if a) the thunderbolt controller is in
secure mode (SL2) and b) the device supports identity verification. If both conditions are true but no key was yet used by userspace to do said identity verification the
key sysfs attribute is empty; NB: the contents of the
key file will always have a trailing newline, even if empty.
bolt_key_load_file so that reading an empty ("" or single whitespace character) files is not an error but will result in
NULL with no
GError being set. Add tests for it.
Closes #121 (closed)