Add negative case for NC sales use tax calculation

[embarnard]

Link to pivotal/JIRA issue

• e.g. https://codeforamerica.atlassian.net/browse/ABCD-123

Is PM acceptance required? (delete one)

• Yes - don't merge until JIRA issue is accepted!

Reminder: merge main into this branch and get green tests before merging to main

What was done?

• Instead of the NC sales use tax calculation returning 0 if the AGI is negative it should return 1; realized this was probably the case after reviewing NJ sales use tax story, here is the thread from that story below:

How to test?

• If AGI is negative for a NC intake then the sales use tax should be 1

[github-actions]

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