Raise error when clone_url_prefix is nil and add specs

app/services/geo/base_sync_service.rb

 module Geo
+  # The clone_url_prefix is used to build URLs for the Geo synchronization
+  # If this is missing from the primary node we raise this exception
+  EmptyCloneUrlPrefixError = Class.new(StandardError)
+
   class BaseSyncService
     class << self
       attr_accessor :type
@@ -71,7 +75,9 @@ module Geo
     end
 
     def primary_ssh_path_prefix
-      @primary_ssh_path_prefix ||= Gitlab::Geo.primary_node.clone_url_prefix
+      @primary_ssh_path_prefix ||= Gitlab::Geo.primary_node.clone_url_prefix.tap do |prefix|
+        raise EmptyCloneUrlPrefixError, 'Missing clone_url_prefix in the primary node' unless prefix.present?
+      end
     end
 
     def log(message)

spec/services/geo/base_sync_service_spec.rb

+require 'spec_helper'
+
+describe Geo::BaseSyncService, services: true do
+  let(:project) { double('project')}
+  subject { described_class.new(project) }
+
+  describe '#primary_ssh_path_prefix' do
+    let!(:primary_node) { create(:geo_node, :primary, host: 'primary-geo-node') }
+
+    it 'raises exception when clone_url_prefix is nil' do
+      allow_any_instance_of(GeoNode).to receive(:clone_url_prefix) { nil }
+
+      expect { subject.send(:primary_ssh_path_prefix) }.to raise_error Geo::EmptyCloneUrlPrefixError
+    end
+
+    it 'returns the prefix defined in the primary node' do
+      expect { subject.send(:primary_ssh_path_prefix) }.not_to raise_error
+      expect(subject.send(:primary_ssh_path_prefix)).to eq('git@localhost:')
+    end
+  end
+end