Fix invalid query syntax when `CategoryCustomField#value` is blank.

app/models/category.rb

@@ -388,9 +388,10 @@ class Category < ActiveRecord::Base
 
   def self.auto_bump_topic!
     bumped = false
+
     auto_bumps = CategoryCustomField
       .where(name: Category::NUM_AUTO_BUMP_DAILY)
-      .where('value::int > 0')
+      .where('NULLIF(value, \'\')::int > 0')
       .pluck(:category_id)
 
     if (auto_bumps.length > 0)
@@ -399,6 +400,7 @@ class Category < ActiveRecord::Base
         break if bumped
       end
     end
+
     bumped
   end
 

spec/models/category_spec.rb

@@ -734,6 +734,11 @@ describe Category do
       category.auto_bump_limiter.clear!
       expect(Category.auto_bump_topic!).to eq(true)
       expect(Topic.where(bumped_at: time).count).to eq(1)
+
+      category.num_auto_bump_daily = ""
+      category.save!
+
+      expect(Category.auto_bump_topic!).to eq(false)
     end
   end