Hola Karamy

You could actually use it in the feminine form "salada" as in "cocina francesa salada" or "cocina francesa dulce" - agreeing with cocina as you said. But we don't normally use it this way because it'd sound a bit odd to classify French cuisine as salty or sweet. 

We are using it in the masculine singular form here to express something like "anything/any dish that is salty [within French cuisine]" or "anything/any dish that is sweet [within French cuisine]" - it is using a more neutral adjective. For this neutral effect the tendency is to use the masculine singular form - this is the reason.

I hope this clarified it.

Saludos

Inma