Normalize the fuel_types table

**Describe the task**
Normalize the fuel_types table to allow for unique records for having nullable percent conifer, percent dead fir and percent grass cure fields.

**Acceptance Criteria**

- [ ] first
- [ ] second
- [ ] third

**Additional context**

- Add any other context about the task here.
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Locate the fuel_types table and inspect how its nullable percent conifer, percent dead fir, and percent grass cure fields currently affect record uniqueness. Define the normalization change needed for unique records, then verify that the three acceptance criteria are satisfied; no file or test is identified in the issue.

Written by the indexing model from the issue text.