Fix return value when Geo nodes are not in sync

Relates to #2469

lib/gitlab/geo/jwt_request_decoder.rb

@@ -42,6 +42,7 @@ module Gitlab
           data
         rescue JWT::DecodeError => e
           Rails.logger.error("Error decoding Geo request: #{e}")
+          return
         end
       end
 

spec/lib/gitlab/geo/jwt_request_decoder_spec.rb

@@ -26,5 +26,11 @@ describe Gitlab::Geo::JwtRequestDecoder do
       primary_node.save
       expect(described_class.new(data).decode).to be_nil
     end
+
+    it 'returns nil when clocks are not in sync' do
+      allow(JWT).to receive(:decode).and_raise(JWT::InvalidIatError)
+
+      expect(subject.decode).to be_nil
+    end
   end
 end