fix(schema) fix errors when encoding default empty arrays

Previously, a schema such as this:
```
{ type = "array",
  default = {},
  elements = { type = "string" },
}
```
would be encoded in json as a `{}` instead of as a `[]` (the Lua table given for the default value would be copied, and its metatable would remain)

This change sets the metatable to `cjson.array_mt` and also adds tests for other possible values given to the default (i.e. `cjson.empty_array`).

kong/db/schema/init.lua

@@ -855,7 +855,11 @@ end
 -- @param field The field definition table.
 local function handle_missing_field(field, value)
   if field.default ~= nil then
-    return tablex.deepcopy(field.default)
+    local copy = tablex.deepcopy(field.default)
+    if field.type == "array" and type(copy) == "table" and not getmetatable(copy) then
+      setmetatable(copy, cjson.array_mt)
+    end
+    return copy
   end
 
   -- If `nilable` (metaschema only), a default value is not necessary.

spec/01-unit/01-db/01-schema/01-schema_spec.lua

@@ -2004,6 +2004,54 @@ describe("schema", function()
       assert.same({ r = { a = "nr", b = 123, }}, data.nested_record)
     end)
 
+    it("detects an empty Lua table as a default for an array and marks it as a json array", function()
+      local Test = Schema.new({
+        fields = {
+          { a = { type = "array",
+                  elements = { type = "string" },
+                  default = {} }, },
+        }
+      })
+      local data = Test:process_auto_fields({})
+      assert.equals('{"a":[]}', cjson.encode(data))
+    end)
+
+    it("accepts cjson.empty_array as a default for an array", function()
+      local Test = Schema.new({
+        fields = {
+          { b = { type = "array",
+                  elements = { type = "string" },
+                  default = cjson.empty_array }, },
+        }
+      })
+      local data = Test:process_auto_fields({})
+      assert.equals('{"b":[]}', cjson.encode(data))
+    end)
+
+    it("accepts a table marked with cjson.empty_array_mt as a default for an array", function()
+      local Test = Schema.new({
+        fields = {
+          { c = { type = "array",
+                  elements = { type = "string" },
+                  default = setmetatable({}, cjson.empty_array_mt) }, },
+        }
+      })
+      local data = Test:process_auto_fields({})
+      assert.equals('{"c":[]}', cjson.encode(data))
+    end)
+
+    it("accepts a table marked with cjson.array_mt as a default for an array", function()
+      local Test = Schema.new({
+        fields = {
+          { d = { type = "array",
+                  elements = { type = "string" },
+                  default = setmetatable({}, cjson.array_mt) }, },
+        }
+      })
+      local data = Test:process_auto_fields({})
+      assert.equals('{"d":[]}', cjson.encode(data))
+    end)
+
     it("nested defaults in required records produce a default record", function()
       local Test = Schema.new({
         fields = {