Rd parser throws error for user macros invoked with empty argument

Bug or feature?

I get the following error from parse_Rd() when a user Rd macro (including system ones) is invoked with empty argument {},
eg \mymacro{}:

Error in tools::parse_Rd(fn) :
   Value of SET_STRING_ELT() must be a 'CHARSXP' not a 'NULL'

A full example is further below with the system macro \CRANpkg{}. In this example it doesn't make sense to use empty argument but the error is not specific to this particular macro.

------
Georgi Boshnakov


Create an Rd file containing system macro \CRANpkg{bibtex}, parse_Rd() is ok:

=================
fn <- tempfile()
writeLines("\\name{dummyfun}
\\alias{dummyfun}
\\title{Dummy title}
\\description{Dummy description}
\\usage{
dummyfun(x, ...)
}
\\arguments{
   \\item{x}{a value.}
   \\item{\\dots}{further arguments.}
}
\\details{
     \\CRANpkg{bibtex}
}
", fn)

tools::parse_Rd(fn)

...
\details{
     \href{{https://CRAN.R-project.org/package=bibtex}{\pkg{bibtex}}}
}

================

With empty argument - \CRANpkg{} - Rd parser throws error. Of course it is silly in this example but I have used it with other user defined  macros where it may make sense to have empty argument.

================

writeLines("\\name{dummyfun}

\\alias{dummyfun}
\\title{Dummy title}
\\description{Dummy description}
\\usage{
dummyfun(x, ...)
}
\\arguments{
   \\item{x}{a value.}
   \\item{\\dots}{further arguments.}
}
\\details{
     \\CRANpkg{}
}
", fn)

tools::parse_Rd(fn)

Error in tools::parse_Rd(fn) :
   Value of SET_STRING_ELT() must be a 'CHARSXP' not a 'NULL'

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