Default Ratio Observations values to null instead of 0

[dczulada]

This is related to

https://oncprojectracking.healthit.gov/support/browse/CYPRESS-2669

The following is from HQMF

2.3.1.3.1Individual Observations
For each case in the DENOM and not in the DENEX, determine the individual DENOM observations.
For each case in the NUMER and not in the NUMEX, determine the individual NUMER observations.

2.3.1.3.2Measusre Aggregates
Using individual observations for all cases in the DENOM and not in the DENEX, calculate the aggregate DENOM.
Using individual observations for all cases in the NUMER and not in the NUMEX, calculate the aggregate NUMER.

Ratio = aggregate NUMER / aggregate DENOM

Pull requests into cqm-execution require the following. Submitter and reviewer should ✅ when done. For items that are not-applicable, note it's not-applicable ("N/A") and ✅.

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• This pull request describes why these changes were made.
• Internal ticket for this PR:
• Internal ticket links to this PR
• Code diff has been done and been reviewed
• Tests are included and test edge cases
• Tests have been run locally and pass

Reviewer 1:

Name:

• Code is maintainable and reusable, reuses existing code and infrastructure where appropriate, and accomplishes the task’s purpose
• The tests appropriately test the new code, including edge cases
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Reviewer 2:

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• Code is maintainable and reusable, reuses existing code and infrastructure where appropriate, and accomplishes the task’s purpose
• The tests appropriately test the new code, including edge cases
• You have tried to break the code

[dczulada]

@adongare I just did a rebase on the most recent master commits. Looks like you might need to approve the PR again.