[SSD1306] prefs start-up optimisation

[DigiH]

Avoid
[ 137][E][Preferences.cpp:483] getString(): nvs_get_str len fail: SSD1306Config NOT_FOUND
error during start-up when no save preferences have been found

lilygo-rtl-433-ssd1306-error

now correctly

************* WELCOME TO OpenMQTTGateway **************
N: SSD1306 config initialised
N: No SSD1306 config to load
N: Setup SSD1306 Display end

and additionally

************* WELCOME TO OpenMQTTGateway **************
N: SSD1306 config initialised
N: Saved SSD1306 config loaded
N: Setup SSD1306 Display end

Checklist:

• The pull request is done against the latest development branch
• Only one feature/fix was added per PR and the code change compiles without warnings
• I accept the DCO.

[1technophile]

Thanks