CB-12218: (android) Fix consistency of null result message

[deton]

Platforms affected

Android

What does this PR do?

Fix problem that JavaScript receives ""(empty string) instead of null
if plugin sends null result from new thread.

What testing has been done on this change?

npm test

Checklist

• Reported an issue in the JIRA database
• Commit message follows the format: "CB-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
• Added automated test coverage as appropriate for this change.

[codecov-io]

Codecov Report

Merging #425 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #425   +/-   ##
=======================================
  Coverage   43.91%   43.91%           
=======================================
  Files          17       17           
  Lines        1710     1710           
  Branches      317      317           
=======================================
  Hits          751      751           
  Misses        959      959

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[infil00p]

LGTM, Merging!