[GOBBLIN-2187] Fix NaN issue while generating size summary

[Blazer-007]

Dear Gobblin 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 the following Gobblin JIRA issues and references them in the PR title. For example, "[GOBBLIN-XXX] My Gobblin PR"
  • https://issues.apache.org/jira/browse/GOBBLIN-2187

Description

• Here are some details about my PR, including screenshots (if applicable):
  when workunit is empty then while serializing wusizesummary GSON throws below exception NaN is not a valid double value as per JSON specification. To override this behavior, use GsonBuilder.serializeSpecialFloatingPointValues() method
  this is coming due to not handling when values are zeros which generates NaN while generating wusizesummary

Tests

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

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"

[phet]

nice work - thank you vivek!