[texinfo.tex 2022-10-18.18]
Consider the following input file
```
\input texinfo.tex
@set txiindexbackslashignore
@findex \\
@findex A
@printindex fn
@bye
```
The created `.fn` file contains
```
\entry{ }{1}{\code {\backslashchar {}\backslashchar {}}}
```
which doesn't make sense, and which produces weird output, see
attached image. Additionally, you get a mysterious pdftex warning
```
pdfTeX warning (dest): name{1} has been referenced but does not exist,
replaced by a fixed one
```
which gives not a single clue where it comes from.
AFAICS, the only method to get a 'space' index entry is having a
series of backslashes if 'txiindexbackslashignore' is set. So maybe
`texinfo.tex` can catch this, producing a correct `\entry` line by
replacing the space in its first argument with a backslash.
If this is not possible, please make `texinfo.tex` suppress index
entries that consist of whitespace only – perhaps together with a
warning (or even an error). I also suggest to add some words to the
manual about this case.
Werner
