Restrict pagination per_page possible values

Sentry error:
https://sentry.gitlab.net/gitlab/gitlabcom/issues/3168349

**Problem**

Providing `0` value for `per_page` causes various internal errors.

**Solution**

* Do not allow to provide 0 for per_page
* Return API error when this happens

Changelog: fixed

lib/api/pagination_params.rb

@@ -18,7 +18,7 @@ module API
       helpers do
         params :pagination do
           optional :page, type: Integer, default: 1, desc: 'Current page number'
-          optional :per_page, type: Integer, default: 20, desc: 'Number of items per page'
+          optional :per_page, type: Integer, default: 20, desc: 'Number of items per page', except_values: [0]
         end
       end
     end

spec/requests/api/commits_spec.rb

@@ -227,6 +227,12 @@ RSpec.describe API::Commits do
             expect(response.headers['X-Page']).to eq('3')
           end
         end
+
+        context 'when per_page is 0' do
+          let(:per_page) { 0 }
+
+          it_behaves_like '400 response'
+        end
       end
 
       context 'with order parameter' do