Improved code, refactored it a bit, implemented full null safety

...a/de/kuschku/libquassel/primitives/QMetaTypeRegistry.java

...hku/libquassel/primitives/serializers/BoolSerializer.java

...bquassel/primitives/serializers/BufferInfoSerializer.java

...ibquassel/primitives/serializers/ByteArraySerializer.java

...hku/libquassel/primitives/serializers/ByteSerializer.java

...hku/libquassel/primitives/serializers/CharSerializer.java

...libquassel/primitives/serializers/DateTimeSerializer.java

...chku/libquassel/primitives/serializers/IntSerializer.java

...hku/libquassel/primitives/serializers/LongSerializer.java

.../libquassel/primitives/serializers/MessageSerializer.java

...ibquassel/primitives/serializers/PrimitiveSerializer.java

...libquassel/primitives/serializers/ProtocolSerializer.java

...ku/libquassel/primitives/serializers/ShortSerializer.java

...bquassel/primitives/serializers/StringListSerializer.java

...u/libquassel/primitives/serializers/StringSerializer.java

...hku/libquassel/primitives/serializers/TimeSerializer.java

...libquassel/primitives/serializers/UserTypeSerializer.java

...quassel/primitives/serializers/VariantListSerializer.java

...bquassel/primitives/serializers/VariantMapSerializer.java

.../libquassel/primitives/serializers/VariantSerializer.java