fix: account for null password.

[RafeArnold]

to reproduce the bug, run the following command from the terminal, then enter Ctrl+D (EOF):

import com.github.ajalt.mordant.terminal.Terminal

fun main() {
  Terminal().readLineOrNull(true)
}

[RafeArnold]

i'm not sure if there's an easy way to test this fix. if there is, please let me know and i'll have a go

[ajalt]

Thank you!