[flagd] Update in-process Evaluation To Allow undefined or null defaultValue

We want to support optional/null defaultValue for flag configurations. The in-process provider must be updated to support this.

Steps

• return FLAG_NOT_FOUND for flag configurations missing a defaultValue and for which targeting doesn't resolve a variant (This is the critical behavioral change)
• add associated gherkin tests from the test harness

This issue requires open-feature/flagd-schemas#186 and open-feature/flagd-testbed#268 to be done first.