browser-sync setOption: allow value to be undefined

[milahu]

otherwise i get TypeError: Cannot read property 'toString' of undefined

sample with patch: when the port is undefined, eleventy will say

[11ty] [Browsersync] Access URLs:
[11ty]  -------------------------------------------
[11ty]        Local: http://localhost:undefined
[11ty]     External: http://192.168.1.120:undefined