_deepmerge(target, source) returns source when source is undefined, rather it should be target.

example:
`
const deepmergeFastify = require('@fastify/deepmerge')({clone: false})
let target: {key: value}
const source: undefined

target = deepmergeFastify(target, source)
// target will be undefined here

But target should be returned as is.
I suspect the issue might be at this line https://github.com/fastify/deepmerge/blob/main/index.js#L136