Warnings fix

[HendrikRauh]

changed 2 things to fix warnings.
Im not sure if this will work like i think, so pls check

[HendrikRauh]

Gateway 0.0.0.0 will be "" then. Gateway 0.0.0.0 should be invalid anyway if im right.

[RaffaelW]

LGTM

[HendrikRauh]

Can we do anything about this warning?

src/main.cpp:54: warning: "ETH_SPI_HOST" redefined
 #define ETH_SPI_HOST SPI2_HOST

In file included from src/main.cpp:9:
lib/AsyncWebServer_ESP32_W5500/src/AsyncWebServer_ESP32_W5500.h:104: note: this is the location of the previous definition
     #define ETH_SPI_HOST            SPI3_HOST

[HendrikRauh]

Also we have this warning:

src/main.cpp: In function 'void setup()':
src/main.cpp:162:55: warning: passing NULL to non-pointer argument 2 of 'uint32_t Preferences::getUInt(const char*, uint32_t)' [-Wconversion-null]
     IPAddress gateway = config.getUInt("gateway", NULL);

It`s the same type i fixed above in this commit.

I am not sure if my approach will break anything, so I will leave it at that. Please have a look at this.