PolicyFromSpec: return nil on error

Returning nil wrapped in Policy interface along with error returns in an
interface with a nil value. This can't be used to perform a nil check.
To make it safe, turn nil value on error.

Signed-off-by: Sunny <darkowlzz@protonmail.com>

internal/policy/factory.go

@@ -38,5 +38,8 @@ func PolicerFromSpec(choice imagev1.ImagePolicyChoice) (Policer, error) {
 		return nil, fmt.Errorf("given ImagePolicyChoice object is invalid")
 	}
 
-	return p, err
+	if err != nil {
+		return nil, err
+	}
+	return p, nil
 }

internal/policy/factory_test.go

@@ -40,4 +40,13 @@ func TestFactory_PolicerFromSpec(t *testing.T) {
 	if err != nil {
 		t.Error("should not return error")
 	}
+
+	// A nil checkable Policer for invalid policy.
+	p, err := PolicerFromSpec(imagev1.ImagePolicyChoice{SemVer: &imagev1.SemVerPolicy{Range: "*-*"}})
+	if err == nil {
+		t.Error("should return error")
+	}
+	if p != nil {
+		t.Error("should be nil")
+	}
 }