Gracefully handle case when no shard data is available

It's possible that if the GeoNodeStatus fails to fetch, we might end
up with a state where the primary loads the last known state without
any shard information. In this case, we should just report "UNKNOWN"
for the storage configuration match check.

Relates to #4586

ee/app/models/geo_node_status.rb

@@ -175,6 +175,7 @@ class GeoNodeStatus < ActiveRecord::Base
   # from a remote node via JSON.
   def storage_shards_match?
     return unless Gitlab::Geo.primary?
+    return unless current_shards && primary_shards
 
     shards_match?(current_shards, primary_shards)
   end

spec/ee/spec/models/geo_node_status_spec.rb

@@ -476,22 +476,24 @@ describe GeoNodeStatus, :geo do
   describe '#storage_shards_match?' do
     before { stub_primary_node }
 
+    set(:status) { create(:geo_node_status) }
+    let(:data) { GeoNodeStatusSerializer.new.represent(status).as_json }
+    let(:result) { described_class.from_json(data) }
+
+    it 'returns nil if no shard data is available' do
+      data.delete('storage_shards')
+
+      expect(result.storage_shards_match?).to be nil
+    end
+
     it 'returns false if the storage shards do not match' do
-      status = create(:geo_node_status)
-      data = GeoNodeStatusSerializer.new.represent(status).as_json
       data['storage_shards'].first['name'] = 'broken-shard'
 
-      result = described_class.from_json(data)
-
       expect(result.storage_shards_match?).to be false
     end
 
     it 'returns true if the storage shards match in different order' do
-      status = create(:geo_node_status)
-
       status.storage_shards.shuffle!
-      data = GeoNodeStatusSerializer.new.represent(status).as_json
-      result = described_class.from_json(data)
 
       expect(result.storage_shards_match?).to be true
     end