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

After commit 1792852f ([symbolizer] Change reaction on invalid
input) llvm-symbolizer issues an error on malformed command instead of
echoing it to the standard output, as in previous versions. It turns out
this behavior broke a use case when echoing was used to check if
llvm-symbolizer is working (https://github.com/llvm/llvm-project/commit/1792852f86dc75efa1f44d46b1a0daf386d64afa#commitcomment-142161925

).

With this change an empty line as input is not considered as an error
anymore and does not produce any output on stderr. llvm-symbolizer still
respond on empty line with line not found, this is consistent with GNU
addr2line.

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Co-authored-by: James Henderson <46713263+jh7370@users.noreply.github.com>