Make default_avatar return a full path.

Callers expect a URL.

This fixes the avatars in commit lists when a user has no avatar and
Gravatar is disabled.

Closes #38715

app/helpers/application_helper.rb

@@ -89,7 +89,7 @@ module ApplicationHelper
   end
 
   def default_avatar
-    'no_avatar.png'
+    asset_path('no_avatar.png')
   end
 
   def last_commit(project)

spec/helpers/application_helper_spec.rb

@@ -100,7 +100,7 @@ describe ApplicationHelper do
       end
 
       it 'returns a generic avatar' do
-        expect(helper.gravatar_icon(user_email)).to match('no_avatar.png')
+        expect(helper.gravatar_icon(user_email)).to match_asset_path('no_avatar.png')
       end
     end
 
@@ -110,7 +110,7 @@ describe ApplicationHelper do
       end
 
       it 'returns a generic avatar when email is blank' do
-        expect(helper.gravatar_icon('')).to match('no_avatar.png')
+        expect(helper.gravatar_icon('')).to match_asset_path('no_avatar.png')
       end
 
       it 'returns a valid Gravatar URL' do