fix(plugins): add len_min=0 for string fields (#12421)

Shorthand fields even though are transient still
use validation. For a string field if it's minimum
length is not defined then the default is 1 to
disallow empty strings.

However for some of the fields underneath shorthand fields
we wanted to allow empty strings therefore we need to add
this len_min=0 to those string fields that can be empty.

KAG-3388

kong/plugins/acme/schema.lua

@@ -41,6 +41,7 @@ local KONG_STORAGE_SCHEMA = {
 local LEGACY_SCHEMA_TRANSLATIONS = {
   { auth = {
     type = "string",
+    len_min = 0,
     func = function(value)
       deprecation("acme: config.storage_config.redis.auth is deprecated, please use config.storage_config.redis.password instead",
         { after = "4.0", })
@@ -57,6 +58,7 @@ local LEGACY_SCHEMA_TRANSLATIONS = {
   }},
   { namespace = {
     type = "string",
+    len_min = 0,
     func = function(value)
       deprecation("acme: config.storage_config.redis.namespace is deprecated, please use config.storage_config.redis.extra_options.namespace instead",
         { after = "4.0", })

kong/plugins/rate-limiting/schema.lua

@@ -119,6 +119,7 @@ return {
           } },
           { redis_password = {
             type = "string",
+            len_min = 0,
             func = function(value)
               deprecation("rate-limiting: config.redis_password is deprecated, please use config.redis.password instead",
                 { after = "4.0", })

kong/plugins/response-ratelimiting/schema.lua

@@ -158,6 +158,7 @@ return {
           } },
           { redis_password = {
             type = "string",
+            len_min = 0,
             func = function(value)
               deprecation("response-ratelimiting: config.redis_password is deprecated, please use config.redis.password instead",
                 { after = "4.0", })