Convert BsonUndefined to null value.

Register a reading converter that returns null when attempting to read a value of type BsonUndefined.
Prior to this change users faced a ConverterNotFoundException when source documents contained BsonUndefined.

Resolves: #2350

Author: christophstrobl

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MongoConverters.java

@@ -33,6 +33,7 @@
 
 import org.bson.BsonReader;
 import org.bson.BsonTimestamp;
+import org.bson.BsonUndefined;
 import org.bson.BsonWriter;
 import org.bson.Document;
 import org.bson.codecs.Codec;
@@ -104,6 +105,7 @@ static Collection<Object> getConvertersToRegister() {
 		converters.add(BinaryToByteArrayConverter.INSTANCE);
 		converters.add(BsonTimestampToInstantConverter.INSTANCE);
 
+		converters.add(reading(BsonUndefined.class, Object.class, it -> null));
 		converters.add(reading(String.class, URI.class, URI::create).andWriting(URI::toString));
 
 		return converters;

Diff for: spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/MappingMongoConverterUnitTests.java

@@ -34,6 +34,7 @@
 import java.time.temporal.ChronoUnit;
 import java.util.*;
 
+import org.bson.BsonUndefined;
 import org.bson.types.Binary;
 import org.bson.types.Code;
 import org.bson.types.Decimal128;
@@ -2843,6 +2844,13 @@ void shouldConvertTypesToStringTargetType() {
 		assertThat(converter.read(Address.class, source).city).isEqualTo("Gotham,Metropolis");
 	}
 
+	@Test // GH-2350
+	void shouldConvertBsonUndefinedToNull() {
+
+		org.bson.Document source = new org.bson.Document("s", "hallway drive").append("city", new BsonUndefined());
+		assertThat(converter.read(Address.class, source).city).isNull();
+	}
+
 	static class GenericType<T> {
 		T content;
 	}