appview/db: cure a few panics

when db.FilterIn(array) is used, the query compiler would panic on empty
lists:

db.GetPipelineStatuses(
s.db,
db.FilterEq("repo_owner", repoInfo.OwnerDid),
db.FilterEq("repo_name", repoInfo.Name),
db.FilterEq("knot", repoInfo.Knot),
db.FilterIn("sha", shas),
)

if `shas` was constructed to be an empty list, it would suffice for the
filter to always fail and subsequently affect zero rows.

Signed-off-by: oppiliappan <[e-mail]>

appview/db/db.go

···
 	// if we have `FilterIn(k, [1, 2, 3])`, compile it down to `k in (?, ?, ?)`
 	if kind == reflect.Slice || kind == reflect.Array {
 		if rv.Len() == 0 {
-			panic(fmt.Sprintf("empty slice passed to %q filter on %s", f.cmp, f.key))
+			// always false
+			return "1 = 0"
 		}
 
 		placeholders := make([]string, rv.Len())
···
 	kind := rv.Kind()
 	if kind == reflect.Slice || kind == reflect.Array {
 		if rv.Len() == 0 {
-			panic(fmt.Sprintf("empty slice passed to %q filter on %s", f.cmp, f.key))
+			return nil
 		}
 
 		out := make([]any, rv.Len())