Fix `difference()` giving extra NA values for short vectors

[mitchelloharawild]

library(tsibble)
#> 
#> Attaching package: 'tsibble'
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, union
difference(1, differences = 3)
#> [1] NA

^{Created on 2024-05-12 with reprex v2.0.2}

Resolves #310