Fix v8_api bugs after testing code in applications

[vmoroz]

This PR fixes two issues in the new v8_api.cpp:

• drainMicrotasks was returning undefined value
• tryGetPreparedScript was retuning non-null even if prepared script is not found in the cache.

  Microsoft Reviewers: codeflow:open?pullrequest=https://github.com/microsoft/v8-jsi/pull/180&drop=dogfoodAlpha