fix(enrich_kv_store_set): default to 0 duration (#151)

Documentation states that this is an optional parameter, but the
initializing function will return an error if it's not set.

> transform: enrich_kv_store_set: time: invalid duration ""

build/config/substation.libsonnet

@@ -412,7 +412,7 @@
           settings: std.prune(std.mergePatch(default, $.helpers.abbv(settings))),
         },
         set(settings={}): {
-          local default = $.transform.enrich.kv_store.default { ttl_key: null, ttl_offset: null },
+          local default = $.transform.enrich.kv_store.default { ttl_key: null, ttl_offset: "0s" },
 
           type: 'enrich_kv_store_set',
           settings: std.prune(std.mergePatch(default, $.helpers.abbv(settings))),

transform/enrich_kv_store_set.go

@@ -89,6 +89,10 @@ func newEnrichKVStoreSet(_ context.Context, cfg config.Config) (*enrichKVStoreSe
 		return nil, fmt.Errorf("transform: enrich_kv_store_set: %v", err)
 	}
 
+	if conf.TTLOffset == "" {
+		conf.TTLOffset = "0s"
+	}
+
 	dur, err := time.ParseDuration(conf.TTLOffset)
 	if err != nil {
 		return nil, fmt.Errorf("transform: enrich_kv_store_set: %v", err)