[llvm] [symbolizer] Empty string is not an error (PR #92660)

[Fangrui Song via llvm-commits]

MaskRay wrote:

"ECHO\n" looks good to me (compatible with pre-1792852f86dc75efa1f44d46b1a0daf386d64afa output). `ECHO` can also be used as command line arguments, e.g.

```
llvm-symbolizer 'DATA 0x2' 'ECHO' 'DATA 0x2' --obj=a.o
```


https://github.com/llvm/llvm-project/pull/92660