Error when computing time_length in years for a missing value

Here an example:

``` r
> time_length(interval(NA,ymd("2016-01-01")))
[1] NA
> time_length(interval(NA,ymd("2016-01-01")), unit = "year")
Error in if (any(e2@year != 0)) { : missing value where TRUE/FALSE needed
```

The second command creates an error when `NA` is expected.
