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Observed that sometimes a VS2017 and a VS2019 directory are present, but
the VS2019 directory is empty (probably because VS2019 was installed then
uninstalled). In this case we should not identify VS2019 as a valid
installation.

Bug: None
Change-Id: I517cf3debc430e949529d06cd5cb281477d5281a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1871990Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709507}