[Android] Avoid unnecessary JNI calls for empty string conversions

There's no need to fetch/release the characters for an empty Java string
when converting it to native. Shortcut such cases by simply returning
an empty C++ string.

BUG=457748
NOTRY=true

Review URL: https://codereview.chromium.org/1263333003

Cr-Commit-Position: refs/heads/master@{#341782}
2 files changed