Don't use nil tests for strings.

[gsmecher]

This has been deprecated since Nim 0.19.0. Quoth the ChangeLog: [1]

nil for strings/seqs is finally gone. Instead the default value for
these is "" / @[]. Use --nilseqs:on for a transition period.

[1] https://github.com/nim-lang/Nim/blob/devel/changelogs/changelog_0_19_0.md