Fix possible division by zero

[Hoiss]

Adafruit_TSL2591::calculateLux function:
It's dangerous in case ch0 is 0!
In case it is 0 use the next greater number 1 for the formula, which should least falsify the calculated result.

[DeqingSun]

I got a lot inf in late night, this fix is necessary.

[caternuson]

Recreated issue using library example:
https://github.com/adafruit/Adafruit_TSL2591_Library/blob/master/examples/tsl2591/tsl2591.ino
and setting gain and integration time to lowest settings.

In dim light with finger over sensor:

[caternuson]

Just to point out, the lux calculation for this sensor is a point of on going discussion:
#14
with the current assessment that it should be taken as a nominal lux value.

Would a better behavior here be just to return 0 for lux if the ch0 value is 0 (or below some low threshold)?

[caternuson]

It also looks like #42 is an attempt to fix this same behavior.

[Hoiss]

Ha, some progress after +2years! Thx @DeqingSun

@caternuson It's....not.....the most frequent discussion in #14 :-D
I think the result is that the current implementation is not good but there is no better one. This is not a good thing.

[caternuson]

OK, we can still improve the current low light behavior. I'd suggest checking the values and returning 0 instead of altering the value so it's not dividing by zero.

[tbattisti]

No progress here?