problem with 'txiindexbackslashignore'

[Werner LEMBERG]

[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