Fix for creating a project through API when import_url is nil

The API was returning 500 when `nil` is passed for the `import_url`.

In fact, it was `Gitlab::UrlSanitizer.valid?` which was throwing a
`NoMethodError` when `nil` value was passed.

changelogs/unreleased/tc-fix-project-create-500.yml

+---
+title: Fix for creating a project through API when import_url is nil
+merge_request: 9841
+author:

lib/gitlab/url_sanitizer.rb

@@ -9,6 +9,8 @@ module Gitlab
     end
 
     def self.valid?(url)
+      return false unless url
+
       Addressable::URI.parse(url.strip)
 
       true

spec/lib/gitlab/url_sanitizer_spec.rb

@@ -70,4 +70,12 @@ describe Gitlab::UrlSanitizer, lib: true do
       expect(sanitizer.full_url).to eq('user@server:project.git')
     end
   end
+
+  describe '.valid?' do
+    it 'validates url strings' do
+      expect(described_class.valid?(nil)).to be(false)
+      expect(described_class.valid?('valid@project:url.git')).to be(true)
+      expect(described_class.valid?('123://invalid:url')).to be(false)
+    end
+  end
 end

spec/requests/api/projects_spec.rb

@@ -424,6 +424,14 @@ describe API::Projects, api: true  do
       expect(json_response['only_allow_merge_if_all_discussions_are_resolved']).to be_truthy
     end
 
+    it 'ignores import_url when it is nil' do
+      project = attributes_for(:project, { import_url: nil })
+
+      post api('/projects', user), project
+
+      expect(response).to have_http_status(201)
+    end
+
     context 'when a visibility level is restricted' do
       let(:project_param) { attributes_for(:project, visibility: 'public') }