fix: omitempty targeting field in Flag structure (open-feature#247)

## This PR
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Added omitempty to the json tags of the `targeting` field in the `Flag`
structure.

### Related Issues
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Fixes open-feature#246 

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Signed-off-by: Skye Gill <gill.skye95@gmail.com>

pkg/eval/json_evaluator_model.go

@@ -68,6 +68,6 @@ type Flag struct {
 	State          string          `json:"state"`
 	DefaultVariant string          `json:"defaultVariant"`
 	Variants       map[string]any  `json:"variants"`
-	Targeting      json.RawMessage `json:"targeting"`
+	Targeting      json.RawMessage `json:"targeting,omitempty"`
 	Source         string          `json:"source"`
 }