`Editor:replaceLines`/`makeModifier` don't handle replacing everything with `nil` properly.

[Possseidon]

The editor assumes that there is always at least one line of (potentially empty) text. Otherwise there would be no place to put the cursor.

However, while implementing Editor:deleteLine(), I noticed that it is very easy to accidentally do just that and I ended up with some odd checks over there. This should probably be handled in makeModifier instead to make it impossible to begin with.

• Add better "nil the entire thing" handling to makeModifier
• Get rid of the (then redundant) workaround in Editor:deleteLine()

[Possseidon]

Technically not a "bug" as I don't think it's possible to get into this scenario with the current implementation, but it's still fairly easy to get into this pitfall (as I noticed with Editor:deleteLine