Fix parsing with invalid timestamps

Sometimes the timestamp in the DSMR message is invalid (when no data
read read from the mbus meter?), and then parsing fails.
Fixing this by handling the exception and returning None for invalid
timestamps. Fixes: #120

test/test_parse_v5.py

@@ -244,3 +244,21 @@ class TelegramParserV5Test(unittest.TestCase):
         corrupted_telegram = TELEGRAM_V5.replace('!6EEE\r\n', '')
         with self.assertRaises(ParseError):
             TelegramParser.validate_checksum(corrupted_telegram)
+
+    def test_gas_timestamp_invalid(self):
+        # Issue 120
+        # Sometimes a MBUS device (For ex a Gas Meter) returns an invalid timestamp
+        # Instead of failing, we should just ignore the timestamp
+        invalid_date_telegram = TELEGRAM_V5.replace(
+            '0-1:24.2.1(170102161005W)(00000.107*m3)\r\n',
+            '0-1:24.2.1(632525252525S)(00000.000)\r\n'
+        )
+        invalid_date_telegram = invalid_date_telegram.replace('!6EEE\r\n', '!90C2\r\n')
+        parser = TelegramParser(telegram_specifications.V5)
+        result = parser.parse(invalid_date_telegram)
+
+        # HOURLY_GAS_METER_READING (0-1:24.2.1)
+        assert isinstance(result[obis.HOURLY_GAS_METER_READING], MBusObject)
+        assert result[obis.HOURLY_GAS_METER_READING].unit is None
+        assert isinstance(result[obis.HOURLY_GAS_METER_READING].value, Decimal)
+        assert result[obis.HOURLY_GAS_METER_READING].value == Decimal('0.000')