base_name is nil when processing ASCIIDOC include

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1739856/?referrer=gitlab_plugin

TypeError: no implicit conversion of nil into String
  gitlab/asciidoc/include_processor.rb:109:in `initialize'
    p = Pathname(base_path)
  gitlab/asciidoc/include_processor.rb:109:in `Pathname'
    p = Pathname(base_path)
  gitlab/asciidoc/include_processor.rb:109:in `resolve_relative_path'
    p = Pathname(base_path)
  gitlab/asciidoc/include_processor.rb:47:in `resolve_target_path'
    path = resolve_relative_path(target, base_path)
  asciidoctor/include_ext/include_processor.rb:48:in `process'
    unless (path = resolve_target_path(target, reader))
...
(211 additional frame(s) were not displayed)

TypeError: asciidoctor: FAILED: <stdin>: Failed to load AsciiDoc document - no implicit conversion of nil into String

TypeError: asciidoctor: FAILED: <stdin>: Failed to load AsciiDoc document - no implicit conversion of nil into String

Issue was refined in #237798 (closed).

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Availability & Testing

Unit tests to reproduce and confirm fix.

Recommended MR Breakdown

1 MR.