Warning when detecting lots of input files

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.
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Description

When we pass a large-ish character to file_in(), we get a warning. All the right files seem to be detected anyway, but we should still remove the causej of the warning.

Reproducible example

library(drake)
plan <- drake_plan(x = file_in(!!letters))
config <- drake_config(plan)
#> Warning in if (nzchar(expr)) {: the condition has length > 1 and only the
#> first element will be used
deps <- deps_target(x, config)
deps <- deps[order(deps$name), ]
head(deps)
#> # A tibble: 6 x 2
#>   name  type   
#>   <chr> <chr>  
#> 1 a     file_in
#> 2 b     file_in
#> 3 c     file_in
#> 4 d     file_in
#> 5 e     file_in
#> 6 f     file_in
tail(deps)
#> # A tibble: 6 x 2
#>   name  type   
#>   <chr> <chr>  
#> 1 u     file_in
#> 2 v     file_in
#> 3 w     file_in
#> 4 x     file_in
#> 5 y     file_in
#> 6 z     file_in

^{Created on 2019-06-05 by the reprex package (v0.3.0)}

Expected output

No warning.

[wlandau]

Fixed in 16c4324.