Treat missing SignedMeterValue observation as no reading

lib/zaptec/state.rb

@@ -36,7 +36,10 @@ def session_identifier
     def operation_mode = charger_operation_mode
 
     def meter_reading
-      @meter_reading ||= MeterReading.parse(@data.fetch(:SignedMeterValue))
+      signed_meter_value = @data[:SignedMeterValue]
+      return if signed_meter_value.blank?
+
+      @meter_reading ||= MeterReading.parse(signed_meter_value)
     end
 
     private

spec/zaptec/state_spec.rb

@@ -18,4 +18,34 @@
       expect(state).not_to be_final_stop_active
     end
   end
+
+  describe "#meter_reading" do
+    it "parses the signed meter value when present" do
+      state = Zaptec::State.new(SignedMeterValue: example_meter_reading)
+
+      expect(state.meter_reading).to have_attributes(reading_kwh: 2935.6)
+    end
+
+    it "is nil when the SignedMeterValue observation is missing from the API response" do
+      state = Zaptec::State.new({})
+
+      expect(state.meter_reading).to be_nil
+    end
+  end
+
+  def example_meter_reading
+    <<~OCMF
+      OCMF|{
+          "FV": "1.0",
+          "RD": [
+              {
+                  "TM": "2018-07-24T13:22:04,000+0200 S",
+                  "RV": 2935.6,
+                  "RU": "kWh",
+                  "ST": "G"
+              }
+          ]
+      }|{"SD":"abc"}
+    OCMF
+  end
 end