Don't change color scheme preference if the value wasn't provided

app/controllers/preferences_controller.rb

@@ -23,7 +23,7 @@ def update
                                     end
 
     site_color_scheme_preference = current_user.preferences.find_or_create_by(:k => "site.color_scheme")
-    site_color_scheme_preference.v = params[:site_color_scheme]
+    site_color_scheme_preference.v = params[:site_color_scheme] if params[:site_color_scheme]
 
     if current_user.save && site_color_scheme_preference.save
       # Use a partial so that it is rendered during the next page load in the correct language.

test/controllers/preferences_controller_test.rb

@@ -22,6 +22,7 @@ def test_routes
 
   def test_update_preferred_editor
     user = create(:user, :languages => [])
+    user.preferences.create(:k => "site.color_scheme", :v => "light")
     session_for(user)
 
     # Changing to a invalid editor should fail
@@ -32,6 +33,7 @@ def test_update_preferred_editor
     assert_select ".alert-success", false
     assert_select ".alert-danger", true
     assert_select "form > div > select#user_preferred_editor > option[selected]", false
+    assert_equal "light", user.preferences.find_by(:k => "site.color_scheme")&.v
 
     # Changing to a valid editor should work
     user.preferred_editor = "id"
@@ -41,6 +43,7 @@ def test_update_preferred_editor
     assert_template :show
     assert_select ".alert-success", /^Preferences updated/
     assert_select "dd", "iD (in-browser editor)"
+    assert_equal "light", user.preferences.find_by(:k => "site.color_scheme")&.v
 
     # Changing to the default editor should work
     user.preferred_editor = "default"
@@ -50,6 +53,7 @@ def test_update_preferred_editor
     assert_template :show
     assert_select ".alert-success", /^Preferences updated/
     assert_select "dd", "Default (currently iD)"
+    assert_equal "light", user.preferences.find_by(:k => "site.color_scheme")&.v
   end
 
   def test_update_preferred_site_color_scheme