[zip] Simplify ZipReader::ExtractCurrentEntryToString()

The output string is not cleared anymore in case of error. This allows
to observe the extracted content even in case of error, which is useful
in unit tests and consistent with the behaviour of the other Extract*()
methods such as ExtractCurrentEntryToFilePathAsync().

BUG=chromium:1295127
TEST=autoninja -C out/Default zlib_unittests && out/Default/zlib_unittests

Change-Id: I498bdd41f9920b6e5b9d62819540476f23979995
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3473545


Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/main@{#973631}
NOKEYCHECK=True
GitOrigin-RevId: da22d8a66a885a419376778e410482678bc9941d