prevent false failure if session state failCondition is undefined

[lwittle]

Dear DIL maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses a bug found during LinkedIn APA-138105

Description

• Here are some details about my PR: When a session state value can be empty, I found that an explicit failure condition needed to be added to ms.session.key.field in order to prevent isSessionStateFailed() from unexpectedly returning true, due to the empty string matching the empty string. This allows the session to continue polling rather than ending after 1 attempt. This case can apply where the session key is the list filenames output (e.g. status check before 2-step file download pattern).

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason: MultistageExtractor does not have unit tests

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. 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
  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"