Wrong logic for initializing _err_sub and _throttle_sub

See this code fragment:
https://github.com/adafruit/Adafruit_IO_Arduino/blob/e2b69fdee825a432c29cf140fdb92ba3a4ea02f7/src/AdafruitIO.cpp#L154-L159

`_err_sub` is only initialized if it's non-NULL, which is the opposite of what I'd expect.
Same for `_throttle_sub`.

As an aside, this will never actually be executed, as both are initialized to `0`.
It doesn't look like this is the intended behavior of this code.

Start in src/AdafruitIO.cpp at lines 154-159 and inspect how _err_sub and _throttle_sub are initialized. Confirm the null checks match the intended initialization behavior, then verify the library still builds successfully.

Written by the indexing model from the issue text.