fix(schema) changes in uuid generation

This commit attempts to solve two problems:
* When updating an entity without including the entity's uuid, the first if would generate an invalid uuid for it (the `if field.uuid ...` branch was not entered, so the `if field.type = "string"` was activated, filling the uuid with a `random_string`

* When converting the old DAO schemas to the new DAO some string values can now be both ngx.null and "". So the auto-generation needs to take that into account and generate also when it encounters "".

kong/db/schema/init.lua

@@ -1233,7 +1233,7 @@ end
 -- for value creation and update.
 -- @param input The table containing data to be processed.
 -- @param context a string describing the CRUD context:
--- valid values are: "insert", "update"
+-- valid values are: "insert", "update", "upsert"
 -- @param nulls boolean: return nulls as explicit ngx.null values
 -- @return A new table, with the auto fields containing
 -- appropriate updated values.
@@ -1247,10 +1247,11 @@ function Schema:process_auto_fields(input, context, nulls)
   for key, field in self:each_field(input) do
 
     if field.auto then
-      if field.uuid and context == "insert" and output[key] == nil then
-        output[key] = utils.uuid()
-      elseif field.uuid and context == "upsert" and output[key] == nil then
-        output[key] = utils.uuid()
+      if field.uuid then
+        if (context == "insert" or context == "upsert") and
+           output[key] == nil or output[key] == "" then
+          output[key] = utils.uuid()
+        end
 
       elseif (key == "created_at" and (context == "insert" or
                                        context == "upsert")) or
@@ -1264,9 +1265,9 @@ function Schema:process_auto_fields(input, context, nulls)
           output[key] = now_s
         end
 
-      elseif field.type == "string" and output[key] == nil
-                                    and (context == "insert" or
-                                         context == "upsert") then
+      elseif field.type == "string" and
+          (context == "insert" or context == "upsert") and
+          (output[key] == nil or output[key] == "") then
         output[key] = utils.random_string()
       end
     end