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

[Serge Pavlov via llvm-commits]

spavloff wrote:

> "ECHO\n" looks good to me (compatible with pre [1792852](https://github.com/llvm/llvm-project/commit/1792852f86dc75efa1f44d46b1a0daf386d64afa) output). `ECHO` can also be used as command line arguments, e.g.
> 
> ```
> llvm-symbolizer 'DATA 0x2' 'ECHO' 'DATA 0x2' --obj=a.o
> ```

If llvm-symbolizer reads input and sees "ECHO", it could be a command `ECHO` or a global symbol named `ECHO`. Commands `CODE`, `DATA` and `FRAME` all have an argument separated by a space. This space allows us to distinguish a command from a symbol. It the case of `ECHO`, there is no such possibility, 

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