Zero-input constructors should return zero-length objects

[hadley]

Rather than an error, as currently:

library(lubridate)

duration()
#> Error in duration(): No valid values have been passed to 'duration' constructor
period()
#> Error in period(): No valid values have been passed to 'period' constructor
interval()
#> Error in tz(start): argument "start" is missing, with no default

This makes more similar in behaviour to base R

integer()
#> integer(0)
factor()
#> factor(0)
#> Levels:

And this property is taken advantage of in vctrs

[hadley]

Would also make more compatible with new POSIXct() and Date() constructors

[vspinu]

Makes sense.