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

[Serge Pavlov via llvm-commits]

spavloff wrote:

> 2. Have a "magic" input string that when received causes llvm-symbolizer to print a specific output string to indicate it is ready.

Actually this change implements this variant. The "maigc" string is just an empty string.

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