Fix direct_messages_max_per_day set to nil

When set to nil, it should mean not zero, but "infinite".

app/models/direct_message.rb

@@ -13,6 +13,7 @@ class DirectMessage < ActiveRecord::Base
   def max_per_day
     return if errors.any?
     max = Setting[:direct_message_max_per_day]
+    return unless max
 
     if sender.direct_messages_sent.today.count >= max.to_i
       errors.add(:title, I18n.t('activerecord.errors.models.direct_message.attributes.max_per_day.invalid'))

spec/models/direct_message_spec.rb

@@ -4,10 +4,6 @@ describe DirectMessage do
 
   let(:direct_message) { build(:direct_message) }
 
-  before(:each) do
-    Setting[:direct_message_max_per_day] = 3
-  end
-
   it "should be valid" do
     expect(direct_message).to be_valid
   end
@@ -33,6 +29,10 @@ describe DirectMessage do
   end
 
   describe "maximum number of direct messages per day" do
+    context "when set" do
+      before(:each) do
+        Setting[:direct_message_max_per_day] = 3
+      end
 
       it "should not be valid if above maximum" do
         sender = create(:user)
@@ -58,7 +58,19 @@ describe DirectMessage do
 
         expect(direct_message).to be_valid
       end
+    end
 
+    context "when unset" do
+      before(:each) do
+        Setting[:direct_message_max_per_day] = nil
+      end
+
+      it "should be valid" do
+        direct_message = build(:direct_message)
+
+        expect(direct_message).to be_valid
+      end
+    end
   end
 
   describe "scopes" do