Validate wifi credentials

[relic-se]

If a device hasn't been configured properly either with CIRCUITPY_WIFI_SSID and CIRCUITPY_WIFI_PASSWORD or the "networks" setting, adafruit_portalbase.network.NetworkBase.connect will result in an obscure TypeError from the adafruit_esp32spi library.

This update filters the adafruit_portalbase.network.NetworkBase._wifi_credentials list to ensure that it includes valid ssid/password pairs. If no valid credentials are available, it will trigger an OSError exception with a descriptive message.

I am testing using the following program written for the Adafruit Fruit Jam with the adafruit_fruitjam library. The device has no WiFi credentials configured.

from adafruit_fruitjam import FruitJam
fruitjam = FruitJam(url="https://www.adafruit.com")
fruitjam.fetch()

Here is the REPL output before this update:

Connecting to AP None
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
  File "adafruit_fruitjam/__init__.py", line 302, in fetch
  File "adafruit_portalbase/network.py", line 552, in fetch
  File "adafruit_portalbase/network.py", line 406, in connect
  File "adafruit_portalbase/wifi_coprocessor.py", line 85, in connect
  File "adafruit_esp32spi/adafruit_esp32spi.py", line 657, in connect
  File "adafruit_esp32spi/adafruit_esp32spi.py", line 680, in connect_AP
  File "adafruit_esp32spi/adafruit_esp32spi.py", line 548, in wifi_set_network
  File "adafruit_esp32spi/adafruit_esp32spi.py", line 421, in _send_command_get_response
  File "adafruit_esp32spi/adafruit_esp32spi.py", line 303, in _send_command
TypeError: object of type 'NoneType' has no len()

Here's the REPL output after this update:

Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
  File "adafruit_fruitjam/__init__.py", line 302, in fetch
  File "/lib/adafruit_portalbase/network.py", line 560, in fetch
  File "/lib/adafruit_portalbase/network.py", line 395, in connect
OSError: No wifi credentials provided

Note: I felt that OSError was appropriate for this exception because it is related to the device configuration before a request is made, but ConnectionError may follow the relative CPython implementation more closely.

[FoamyGuy]

This looks good to me. Thanks for working on it!