[feature request(collections)] option to merge undefined in deepMerge #4752

Is your feature request related to a problem? Please describe.

when merging together deeply nested object (e.g react state), it's important to get 'nonempty' values when merging 'empty' values with 'nonempty' values.

for array, map and set it defaults to merging empty and non-empty values together:

deepMerge({ a: new Set([1]) }, { a: new Set() })
//=> { a: Set(1) { 1 } }

however, it doesn't work with nullish values. it'd be very useful to be able to output non-nullish value from merging nullish and non-nullish value together.

deepMerge({ a: 1 }, { a: undefined })
//=> { a: undefined }
// expected: { a: 1 }

Describe the solution you'd like

in DeepMergeOptions, add nullish: MergingStrategy that controls whether deepMerge will choose non-nullish value over nullish ones, such that

type State = {
  pos: {
    x: number
    y: number
  }
}
type UpdateState = Partial<{
  pos: Partial<{ 
    x: number
    y: number 
  }>
}>

const merge = (state: State, newState: UpdateState): State => deepMerge(state, newState, { nullish: "merge" })

always holds. (currently the return type of merge will be UpdateState)

Describe alternatives you've considered

make nullish: "merge” as default. this may cause breaking changes.