Handle nil for config.govuk_time_zone

lib/govuk_app_config/govuk_timezone.rb

@@ -2,7 +2,7 @@ module GovukTimezone
   def self.configure(config)
     raise "govuk_app_config prevents configuring time_zone with config.time_zone - use config.govuk_time_zone instead" unless config.time_zone == "UTC"
 
-    if config.respond_to? :govuk_time_zone
+    if config.respond_to?(:govuk_time_zone) && config.govuk_time_zone.present?
       config.time_zone = config.govuk_time_zone
     else
       Rails.logger.info 'govuk_app_config changing time_zone from UTC (the rails default) to London (the GOV.UK default). Set config.govuk_time_zone = "UTC" if you need UTC.'

spec/lib/govuk_timezone_spec.rb

@@ -22,6 +22,13 @@
       expect(config.time_zone).to eq("Shanghai")
     end
 
+    it "should default to London if config.govuk_time_zone is nil" do
+      config = Struct.new(:time_zone, :govuk_time_zone).new("UTC", nil)
+      expect(logger).to receive(:info)
+      GovukTimezone.configure(config)
+      expect(config.time_zone).to eq("London")
+    end
+
     it "should raise an error if config.time_zone is set to anything other than the default UTC" do
       config = Struct.new(:time_zone).new("London")
       expect { GovukTimezone.configure(config) }.to raise_error(/govuk_app_config prevents configuring time_zone with config[.]time_zone/)