…iber#2369)" (gofiber#2742)"

This reverts commit 28be17f.

Should check for `nil` since it is `any`.

`context` recommends using "unexported type" as context keys to avoid
collisions https://pkg.go.dev/github.com/gofiber/fiber/v2#Ctx.Locals.

The official go blog also recommends this https://go.dev/blog/context.

`fiber.Ctx.Locals(key any, value any)` correctly allows consumers to
use unexported types or e.g. strings.

But some fiber middlewares constrain their context-keys to `string` in
their "default config structs", making it impossible to use unexported
types.

This PR removes the `string` _constraint_ from all middlewares, allowing
to now use unexported types as per the official guidelines. However
the default value is still a string, so it's not a breaking change, and
anyone still using strings as context keys is not affected.