fix(smartplaylist): don't let invalid rules hide all playlists - #6316

Empty nested groups were saved as {} and could not be read back, failing
every playlist query. Drop them while parsing, and log and skip unreadable
rules stored in the DB instead of failing the whole listing.

Signed-off-by: DualFroz <me@dualfroz.com>

model/criteria/criteria_test.go

@@ -103,6 +103,31 @@ var _ = Describe("Criteria", func() {
 		)
 	})
 
+	Context("with empty groups", func() {
+		DescribeTable("rejects criteria with no rules other than empty groups",
+			func(jsonStr string) {
+				var c Criteria
+				err := json.Unmarshal([]byte(jsonStr), &c)
+				gomega.Expect(err).To(gomega.MatchError(gomega.ContainSubstring("missing rules")))
+			},
+			Entry("empty object", `{"all":[{}],"sort":"+lastplayed","limit":10}`),
+			Entry("empty nested all", `{"all":[{"all":[]}],"sort":"+lastplayed","limit":10}`),
+			Entry("empty nested any", `{"any":[{"any":[]}]}`),
+			Entry("nested group holding an empty object", `{"all":[{"any":[{}]}]}`),
+		)
+
+		It("drops empty nested groups, so the rules can be read back after saving", func() {
+			var c Criteria
+			err := json.Unmarshal([]byte(`{"all":[{"is":{"loved":true}},{"any":[]}]}`), &c)
+			gomega.Expect(err).ToNot(gomega.HaveOccurred())
+			gomega.Expect(c.Expression).To(gomega.Equal(All{Is{"loved": true}}))
+
+			j, err := json.Marshal(c)
+			gomega.Expect(err).ToNot(gomega.HaveOccurred())
+			gomega.Expect(string(j)).To(gomega.Equal(`{"all":[{"is":{"loved":true}}]}`))
+		})
+	})
+
 	Describe("LimitPercent", func() {
 		Describe("JSON round-trip", func() {
 			It("marshals and unmarshals limitPercent", func() {