Using tilde `~` (null) in antora.yml should not produce a warning

When using the following in antora.yml:

name: admin
title: Administration
version: 'master'
nav:
  - modules/ROOT/nav.adoc
asciidoc:
  attributes:
    toc: ~

I get the following warning message:

AsciiDoc attribute 'toc' is null in /path/to/antora.yml: Will be treated as unset. Use either false to explicitly unset, or set by providing for example an empty string ''

According to Dan:

that is the correct syntax. valid values are false, nil (also written as ~), or any of the enumerated values. in a playbook, using false and nil are preferred over using !toc (since it’s cleaner and easier to control keys) see https://docs.antora.org/antora/2.3/playbook/asciidoc-attributes/#hard-unset

The message is a bit misleading and I think we should not warn when the value is explicitly null (~).

[ahus1]

Will be part of 0.31.38. Waiting for TravisCI to build the release.

[ahus1]

Thanks for reporting this.

Now I know something more about YAML: When there is a ~ without quotes, the parser will present it to the code as null.

Will remove the corresponding lines in AsciiDoc.java for the next release.