Cleanup acc. to review remarks

src/devices/Ahtxx/AhtBase.cs

@@ -168,11 +168,6 @@ namespace Iot.Device.Ahtxx
         /// <inheritdoc cref="IDisposable" />
         protected virtual void Dispose(bool disposing)
         {
-            if (_i2cDevice == null)
-            {
-                return;
-            }
-
             _i2cDevice?.Dispose();
             _i2cDevice = null;
         }

src/devices/Mhz19b/Mhz19b.cs

@@ -66,10 +66,12 @@ namespace Iot.Device.Mhz19b
             }
 
             // create serial port using the setting acc. to datasheet, pg. 7, sec. general settings
-            _serialPort = new SerialPort(uartDevice, 9600, Parity.None, 8, StopBits.One);
-            _serialPort.Encoding = Encoding.ASCII;
-            _serialPort.ReadTimeout = 1000;
-            _serialPort.WriteTimeout = 1000;
+            _serialPort = new SerialPort(uartDevice, 9600, Parity.None, 8, StopBits.One)
+            {
+                Encoding = Encoding.ASCII,
+                ReadTimeout = 1000,
+                WriteTimeout = 1000
+            };
             _serialPort.Open();
             _serialPortStream = _serialPort.BaseStream;
             _shouldDispose = true;