[BUG] Can't export variable named "nil" to Texinfo

[8dcc <8dcc.git@gmail.com>]

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Hello,

I can export the following "Variable" to Texinfo with
`org-texinfo-export-to-texinfo':

  - Variable: my-name ::

    Content...

Results in the following .texi:

  @defvar my-name
  Content@dots{}
  @end defvar

However, if the variable is named "nil" (without the quotes), the
variable name is not exported, and a warning is generated when
processing the .texi file:

  - Variable: nil ::

    Content...

Results in:

  @defvar
  Content@dots{}
  @end defvar

When running `makeinfo --pdf file.texi':

  file.texi:50: warning: missing category for @defvar

I saw the "Variable" to "@defvar" conversion is made with
`org-texinfo--definition-command-alist' in `ox-texinfo.el', which then
gets used in `org-texinfo--match-definition' through the
regexp. However, I was not able to see why a "nil" string isn't
processed.

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