Allow visitors to return undefined
Allow visitors to return undefined

While implementing `ts.Visitor`, it is possible to return `undefined` in order to drop a node. However, the typings do not reflect this and only allow to return `Node | Node []`.

This PR extends the typings to allow `undefined` as well.
kpreisser/TypeScript,synaptek/TypeScript,SaschaNaz/TypeScript,basarat/TypeScript,Microsoft/TypeScript,donaldpipowitch/TypeScript,chuckjaz/TypeScript,microsoft/TypeScript,basarat/TypeScript,RyanCavanaugh/TypeScript,minestarks/TypeScript,synaptek/TypeScript,nojvek/TypeScript,weswigham/TypeScript,donaldpipowitch/TypeScript,donaldpipowitch/TypeScript,Microsoft/TypeScript,basarat/TypeScript,weswigham/TypeScript,kitsonk/TypeScript,RyanCavanaugh/TypeScript,RyanCavanaugh/TypeScript,TukekeSoft/TypeScript,minestarks/TypeScript,chuckjaz/TypeScript,alexeagle/TypeScript,kitsonk/TypeScript,chuckjaz/TypeScript,microsoft/TypeScript,basarat/TypeScript,alexeagle/TypeScript,chuckjaz/TypeScript,Eyas/TypeScript,Microsoft/TypeScript,donaldpipowitch/TypeScript,weswigham/TypeScript,kitsonk/TypeScript,SaschaNaz/TypeScript,nojvek/TypeScript,SaschaNaz/TypeScript,Eyas/TypeScript,synaptek/TypeScript,kpreisser/TypeScript,nojvek/TypeScript,synaptek/TypeScript,nojvek/TypeScript,microsoft/TypeScript,alexeagle/TypeScript,kpreisser/TypeScript,SaschaNaz/TypeScript,minestarks/TypeScript,TukekeSoft/TypeScript,Eyas/TypeScript,TukekeSoft/TypeScript,Eyas/TypeScript