Polishing.

Add missing Nullable annotations to serializers.

See #3053
Original pull request: #3058

src/main/java/org/springframework/data/redis/serializer/GenericJackson2JsonRedisSerializer.java

@@ -271,6 +271,7 @@ public byte[] serialize(@Nullable Object value) throws SerializationException {
 	}
 
 	@Override
+	@Nullable
 	public Object deserialize(@Nullable byte[] source) throws SerializationException {
 		return deserialize(source, Object.class);
 	}

src/main/java/org/springframework/data/redis/serializer/GenericToStringSerializer.java

@@ -92,6 +92,7 @@ public byte[] serialize(@Nullable T value) {
 	}
 
 	@Override
+	@Nullable
 	public T deserialize(@Nullable byte[] bytes) {
 
 		if (bytes == null) {

src/main/java/org/springframework/data/redis/serializer/Jackson2JsonRedisSerializer.java

@@ -158,6 +158,7 @@ public byte[] serialize(@Nullable T value) throws SerializationException {
 		}
 	}
 
+	@Nullable
 	@Override
 	@SuppressWarnings("unchecked")
 	public T deserialize(@Nullable byte[] bytes) throws SerializationException {

src/main/java/org/springframework/data/redis/serializer/JdkSerializationRedisSerializer.java

@@ -84,6 +84,7 @@ public JdkSerializationRedisSerializer(Converter<Object, byte[]> serializer,
 		this.deserializer = deserializer;
 	}
 
+	@Nullable
 	@Override
 	public byte[] serialize(@Nullable Object value) {
 
@@ -98,6 +99,7 @@ public byte[] serialize(@Nullable Object value) {
 		}
 	}
 
+	@Nullable
 	@Override
 	public Object deserialize(@Nullable byte[] bytes) {
 

src/main/java/org/springframework/data/redis/serializer/OxmSerializer.java

@@ -102,6 +102,7 @@ public byte[] serialize(@Nullable Object value) throws SerializationException {
 		return stream.toByteArray();
 	}
 
+	@Nullable
 	@Override
 	public Object deserialize(@Nullable byte[] bytes) throws SerializationException {
 

src/main/java/org/springframework/data/redis/serializer/StringRedisSerializer.java

@@ -80,11 +80,13 @@ public StringRedisSerializer(Charset charset) {
 		this.charset = charset;
 	}
 
+	@Nullable
 	@Override
 	public byte[] serialize(@Nullable String value) {
 		return (value == null ? null : value.getBytes(charset));
 	}
 
+	@Nullable
 	@Override
 	public String deserialize(@Nullable byte[] bytes) {
 		return (bytes == null ? null : new String(bytes, charset));