`isMultiple` should result in an empty array when the flag is not specified

[sindresorhus]

Currently, it returns undefined, which is incorrect.

I had to work around it here: sindresorhus/replace-in-files-cli@e9ac881#diff-2cce40143051e25f811b56c79d619bf5R63

// @ulken

[felizj17]

I would like to give this a go

[felizj17]

wait nvm i mightve asked for more than i can chew, ill just wait and see how this is solved, sorry just starting out

[ulken]

Noted. I'll have a look 👀 in the coming days if nobody picks it up before that.

[ulken]

Took a stab at it