Adapt to snapshot change in Spring Data MongoDB

See spring-projects/spring-data-mongodb#5054
See spring-projectsgh-47041

Author: snicoll

module/spring-boot-data-mongodb/src/main/java/org/springframework/boot/data/mongodb/autoconfigure/DataMongoProperties.java

@@ -103,7 +103,7 @@ public static class Representation {
 		/**
 		 * Representation to use when converting a BigDecimal.
 		 */
-		private @Nullable BigDecimalRepresentation bigDecimal;
+		private @Nullable BigDecimalRepresentation bigDecimal = BigDecimalRepresentation.UNSPECIFIED;
 
 		public @Nullable BigDecimalRepresentation getBigDecimal() {
 			return this.bigDecimal;