fix: server union type contains null

[jannyHou]

The address() method from net module has null in its union type, which results in build error in the http-server module. See pic 1:

The type definition of address from net module:

I am not very familiar with the address resolve, so treated null as an empty string here. Should we throw error if it's null?

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[jannyHou]

Closing.
@types/[email protected] passes: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/node/v10/net.d.ts#L59

Only @types/[email protected] fails:
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/node/net.d.ts#L176