DATAMONGO-2385 - Polishing.

Remove null checks for arguments known to be not-null. Remove Nullable annotations from methods that never return null.

Original pull request: #802.

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

@@ -211,7 +211,7 @@ enum URLToStringConverter implements Converter<URL, String> {
 		INSTANCE;
 
 		public String convert(URL source) {
-			return source == null ? null : source.toString();
+			return source.toString();
 		}
 	}
 
@@ -224,7 +224,7 @@ enum StringToURLConverter implements Converter<String, URL> {
 		public URL convert(String source) {
 
 			try {
-				return source == null ? null : new URL(source);
+				return new URL(source);
 			} catch (MalformedURLException e) {
 				throw new ConversionFailedException(SOURCE, TARGET, source, e);
 			}
@@ -483,7 +483,6 @@ enum BinaryToByteArrayConverter implements Converter<Binary, byte[]> {
 
 		INSTANCE;
 
-		@Nullable
 		@Override
 		public byte[] convert(Binary source) {
 			return source.getData();
@@ -501,7 +500,6 @@ enum BsonTimestampToInstantConverter implements Converter<BsonTimestamp, Instant
 
 		INSTANCE;
 
-		@Nullable
 		@Override
 		public Instant convert(BsonTimestamp source) {
 			return Instant.ofEpochSecond(source.getTime(), 0);