Fix redis empty category

[madafoo]

When setting auto_insert_item = true, user feedback with a nonexist itemid will generate a empty record in the database, this action will cause bellow panic

Apr 27 01:01:21 test gorse-in-one[1304502]: 2025/04/27 01:01:21 http: panic serving 192.168.0.2:54280: runtime error: slice bounds out of range [1:0]
Apr 27 01:01:21 test gorse-in-one[1304502]: goroutine 13846 [running]:
Apr 27 01:01:21 test gorse-in-one[1304502]: net/http.(*conn).serve.func1()
Apr 27 01:01:21 test gorse-in-one[1304502]:         net/http/server.go:1947 +0xbe
Apr 27 01:01:21 test gorse-in-one[1304502]: panic({0x5a172a417f20?, 0xc000176768?})
Apr 27 01:01:21 test gorse-in-one[1304502]:         runtime/panic.go:792 +0x132
Apr 27 01:01:21 test gorse-in-one[1304502]: github.com/zhenghaoz/gorse/storage/cache.decodeCategory({0x0?, 0x0?})
Apr 27 01:01:21 test gorse-in-one[1304502]:         github.com/zhenghaoz/gorse/storage/cache/redis.go:478 +0x88
Apr 27 01:01:21 test gorse-in-one[1304502]: github.com/zhenghaoz/gorse/storage/cache.decodeCategories({0x0?, 0x1?})
Apr 27 01:01:21 test gorse-in-one[1304502]:         github.com/zhenghaoz/gorse/storage/cache/redis.go:495 +0x91
Apr 27 01:01:21 test gorse-in-one[1304502]: github.com/zhenghaoz/gorse/storage/cache.(*Redis).SearchScores(0xc0005e7560, {0x5a172a535188, 0xc0008753e0}, {0x5a17286cb052?,>
Apr 27 01:01:21 test gorse-in-one[1304502]:         github.com/zhenghaoz/gorse/storage/cache/redis.go:316 +0x82e
Apr 27 01:01:21 test gorse-in-one[1304502]: github.com/zhenghaoz/gorse/server.(*RestServer).SearchDocuments(0xc000666788, {0x5a17286cb052, 0xc}, {0xc00179a0e0, 0xb}, {0x0>
Apr 27 01:01:21 test gorse-in-one[1304502]:         github.com/zhenghaoz/gorse/server/rest.go:653 +0x38a
Apr 27 01:01:21 test gorse-in-one[1304502]: github.com/zhenghaoz/gorse/master.(*Master).getItemToItem(0xc000666788, 0xc00083a000, 0xc000690070)
Apr 27 01:01:21 test gorse-in-one[1304502]:         github.com/zhenghaoz/gorse/master/rest.go:863 +0x1b2
Apr 27 01:01:21 test gorse-in-one[1304502]: github.com/emicklei/go-restful/v3.(*FilterChain).ProcessFilter(0xc000718000?, 0x5a172a535188?, 0xc0008753e0?)

and also this one

Apr 27 01:58:09 test gorse-in-one[1429087]: 2025/04/27 01:58:09 http: panic serving 192.168.0.2:51256: runtime error: index out of range [0] with length 0
Apr 27 01:58:09 test gorse-in-one[1429087]: goroutine 574 [running]:
Apr 27 01:58:09 test gorse-in-one[1429087]: net/http.(*conn).serve.func1()
Apr 27 01:58:09 test gorse-in-one[1429087]:         net/http/server.go:1947 +0xbe
Apr 27 01:58:09 test gorse-in-one[1429087]: panic({0x5d3777f7af20?, 0xc002d5fb48?})
Apr 27 01:58:09 test gorse-in-one[1429087]:         runtime/panic.go:792 +0x132
Apr 27 01:58:09 test gorse-in-one[1429087]: github.com/zhenghaoz/gorse/server.(*RestServer).getTypedUserItemFeedback(0xc000672788, 0xc002d7ebe0, 0xc000752bd0)
Apr 27 01:58:09 test gorse-in-one[1429087]:         github.com/zhenghaoz/gorse/server/rest.go:1878 +0x245
Apr 27 01:58:09 test gorse-in-one[1429087]: github.com/emicklei/go-restful/v3.(*FilterChain).ProcessFilter(0xc0006617c0?, 0x5d3778098188?, 0xc002d6aae0?)
Apr 27 01:58:09 test gorse-in-one[1429087]:         github.com/emicklei/go-restful/[email protected]/filter.go:23 +0x53
Apr 27 01:58:09 test gorse-in-one[1429087]: go.opentelemetry.io/contrib/instrumentation/github.com/emicklei/go-restful/otelrestful.OTelFilter.func1(0xc002d7ebe0, 0xc00075>
Apr 27 01:58:09 test gorse-in-one[1429087]:         go.opentelemetry.io/contrib/instrumentation/github.com/emicklei/go-restful/[email protected]/restful.go:65 +0x46f
Apr 27 01:58:09 test gorse-in-one[1429087]: github.com/emicklei/go-restful/v3.(*FilterChain).ProcessFilter(0x200000003?, 0x0?, 0x0?)
Apr 27 01:58:09 test gorse-in-one[1429087]:         github.com/emicklei/go-restful/[email protected]/filter.go:21 +0x42
Apr 27 01:58:09 test gorse-in-one[1429087]: github.com/zhenghaoz/gorse/server.(*RestServer).MetricsFilter(0x148?, 0xc002d7ebe0, 0xc000752bd0, 0xc002c7fdb0)
Apr 27 01:58:09 test gorse-in-one[1429087]:         github.com/zhenghaoz/gorse/server/rest.go:161 +0x66

[zhenghaoz]

Empty category is _. You might represent "real" empty category by error or another return value.

[madafoo]

The dashboard just use the '' as the empty category, "_" is a internal value just used in the redis code. I think just return '' as the empty category is ok for now.
https://github.com/gorse-io/dashboard/blob/ed2e9e24e42b80e227a25dba70c48eb385cff91b/src/views/Item.vue#L199