Continue task pipeline if currentState is null

[alirezazamani]

Issues

• My PR addresses the following Helix issues and references them in the PR description:
  Fixes Null currentState cause the TASK pipeline to be stuck #1322

Description

• Here are some details about my PR, including screenshots of any UI changes:
  In this PR, if controller encounters null currentStates, it will continue the pipeline and TaskSchedulingStage. Also such currentState would not count toward the quota.

Tests

• The following tests are written for this issue:
  TestTaskCurrentStateNull

• The following is the result of the "mvn test" command on the appropriate module:

Helix-Core:

[INFO] Results:
[INFO] 
[ERROR] Failures: 
[ERROR]   TestEnableCompression.testEnableCompressionResource:117 expected:<true> but was:<false>
[ERROR]   TestEnqueueJobs.testQueueParallelJobs » ThreadTimeout Method org.testng.intern...
[INFO] 
[ERROR] Tests run: 1175, Failures: 2, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  01:22 h
[INFO] Finished at: 2020-08-28T11:46:15-07:00
[INFO] ------------------------------------------------------------------------

The failed test passed when run individually.
mvn test -Dtest="TestEnableCompression,TestEnqueueJobs"

[INFO] Tests run: 6, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 79.623 s - in TestSuite
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 6, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  01:25 min
[INFO] Finished at: 2020-08-28T11:48:05-07:00
[INFO] ------------------------------------------------------------------------

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)

[NealSun96]

LGTM

[narendly]

@alirezazamani could you update with your final commit message?

[alirezazamani]

This PR is ready to be merged.

Final commit message:
Continue pipeline if currentState is null

In this commit, if controller encounters null currentStates, it will
continue the pipeline and TaskSchedulingStage. Also such
currentState would not count toward the quota.