Fix bug in str_trunc()

[UchidaMizuki]

I found a bug in str_trunc() where strings are not properly truncated when width is small, as shown below.
The bug seems to be caused by str_sub("abc", 0, -1) outputting "abc" instead of "".
I have fixed the bug by changing the behaviour of getting the tail of the strings.

library(stringr)

trunc <- function(direction, width) str_trunc(
  "This string is moderately long",
  direction,
  width = width
)

trunc("left", 3)
#> [1] "...This string is moderately long"
trunc("center", 3)
#> [1] "...This string is moderately long"
trunc("center", 4)
#> [1] "T...This string is moderately long"

str_sub("abc", 0, -1)
#> [1] "abc"
# => Not equal to ""

^{Created on 2023-01-14 with reprex v2.0.2}

[UchidaMizuki]

Close this pull request temporarily as I'm starting to think that stringi::stri_sub() might be misbehaving.

[UchidaMizuki]

Furthermore, the use of ifelse() is not considered appropriate because it is not vectorized.