gpio: uapi: clarify the meaning of 'empty' char arrays

Clarify that a char array containing a string is considered 'empty' if
the first character is the null terminator. The remaining characters
are not relevant to this determination.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20201005070329.21055-6-warthog618@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>