fix(compiler-core): compilation of slot v-if/else when preserve whitespace

[pikax]

closes #6063 based on https://github.com/vuejs/core/pull/6124

Not exposing the whitespace option to the context because I don't think

// pseudo-node
SlotNode(vIf)
TextNode('')
SlotNode(vElse)

Is possible without the whitespace preserve, since it check for text content to be empty