Intermittent failure when building frontend integration test.

Below is the failure message.

```
:ambry-frontend:processUnitTestResources UP-TO-DATE
:ambry-frontend:unitTestClasses
:ambry-frontend:unitTest
com.github.ambry.frontend.FrontendIntegrationTest > multipartPostGetHeadTest[3] FAILED
java.lang.NullPointerException
at com.github.ambry.frontend.FrontendIntegrationTest.getBlobInfoAndVerify(FrontendIntegrationTest.java:576)
at com.github.ambry.frontend.FrontendIntegrationTest.doPostGetHeadDeleteTest(FrontendIntegrationTest.java:388)
at com.github.ambry.frontend.FrontendIntegrationTest.multipartPostGetHeadTest(FrontendIntegrationTest.java:191)
74 tests completed, 1 failed
:ambry-frontend:unitTest FAILED
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':ambry-frontend:unitTest'.
> There were failing tests. See the report at: file:///home/travis/build/linkedin/ambry/ambry-frontend/build/reports/tests/index.html
* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.
BUILD FAILED
Total time: 49.581 secs
The command "./gradlew build && ./gradlew codeCoverageReport" exited with 1.
before_cache.1
0.00s$ rm -f $HOME/.gradle/caches/modules-2/modules-2.lock
before_cache.2
0.00s$ rm -fr $HOME/.gradle/caches/*/plugin-resolution/
cache.2
store build cache
0.00s
1.42snothing changed, not updating cache
Done. Your build exited with 1.
```

Start with ambry-frontend/src test code for FrontendIntegrationTest, especially multipartPostGetHeadTest, doPostGetHeadDeleteTest, and getBlobInfoAndVerify at line 576. Reproduce the failure with ./gradlew build and inspect the referenced test report or stack trace. Done means the intermittent NullPointerException is addressed and the frontend unit test suite completes successfully.

Written by the indexing model from the issue text.