Group some if conditions from Server:initialize`

The conditions like

    if a == nil and b == nil then
        c = 'something'
    end
    if a == nil and b then
        c = 'something else'
    end

going in a row look a little strange.

This patch groups similar conditions from the `Server:initialize`
function into the following construction:

    if a == nil then
        if b == nil then
            c = 'something'
        else
            c = 'something else'
        end
    end

luatest/server.lua

@@ -147,11 +147,12 @@ function Server:initialize()
         self.http_client = http_client.new()
     end
 
-    if self.net_box_uri == nil and self.net_box_port == nil then
-        self.net_box_uri = self.build_listen_uri(self.alias, self.rs_id or self.id)
-    end
-    if self.net_box_uri == nil and self.net_box_port then
-        self.net_box_uri = 'localhost:' .. self.net_box_port
+    if self.net_box_uri == nil then
+        if self.net_box_port == nil then
+            self.net_box_uri = self.build_listen_uri(self.alias, self.rs_id or self.id)
+        else
+            self.net_box_uri = 'localhost:' .. self.net_box_port
+        end
     end
 
     self.env = utils.merge(self.env or {}, self:build_env())