drivers: wifi: esp_at: fix connect to open network

According to ESP-AT documentation ([1] for version before 2.0 and [2]
for version 2.1) of AT+CWJAP command, both SSID and PSK are required.
Even for newest ESP-AT release 2.2 ([3]) "," (comma) is needed even if
SSID or PSK are not explicitly provided.

Send 'AT+CWJAP="SSID",""' instead of 'AT+CWJAP="SSID"' when connecting
to open WiFi network, to follow AT commands documentation.

Tested with ESP-AT firmware 2.1.

[1] https://www.espressif.com/sites/default/files/documentation/4a-esp8266_at_instruction_set_en.pdf
[2] https://github.com/espressif/esp-at/blob/release/v2.1.0.0_esp8266/docs/en/AT_Command_Set/Wi-Fi_AT_Commands.md#atcwjapconnects-to-an-ap
[3] https://github.com/espressif/esp-at/blob/release/v2.2.0.0_esp8266/docs/en/AT_Command_Set/Wi-Fi_AT_Commands.rst#refatcwjap-wifi-at-connect-to-an-ap



Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>