Don't keep default theme name as empty string

app/models/asciicast.rb

@@ -81,6 +81,13 @@ class Asciicast < ActiveRecord::Base
     value ? super(value.strip[0...255]) : super
   end
 
+  def theme_name=(value)
+    if value == ""
+      value = nil
+    end
+    super(value)
+  end
+
   def self.generate_secret_token
     SecureRandom.hex.to_i(16).to_s(36).rjust(25, '0')
   end
@@ -126,7 +133,7 @@ class Asciicast < ActiveRecord::Base
   end
 
   def theme
-    theme_name.presence && Theme.for_name(theme_name)
+    theme_name && Theme.for_name(theme_name)
   end
 
   def image_filename

app/models/user.rb

@@ -79,8 +79,15 @@ class User < ActiveRecord::Base
     value ? super(value.strip) : super
   end
 
+  def theme_name=(value)
+    if value == ""
+      value = nil
+    end
+    super(value)
+  end
+
   def theme
-    theme_name.presence && Theme.for_name(theme_name)
+    theme_name && Theme.for_name(theme_name)
   end
 
   def assign_api_token(token)

db/migrate/20170215120234_convert_empty_string_theme_to_nil.rb

@@ -0,0 +1,6 @@
+class ConvertEmptyStringThemeToNil < ActiveRecord::Migration
+  def change
+    execute "UPDATE users SET theme_name=NULL WHERE theme_name=''"
+    execute "UPDATE asciicasts SET theme_name=NULL WHERE theme_name=''"
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20150510164222) do
+ActiveRecord::Schema.define(version: 20170215120234) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"