Improve null-safety of module/spring-boot-mongodb

See gh-46926

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

@@ -117,8 +117,9 @@ public final class MongoReactiveAutoConfiguration {
 		@Override
 		public void customize(Builder builder) {
 			if (!isCustomTransportConfiguration(this.settings.getIfAvailable())) {
-				this.eventLoopGroup = new MultiThreadIoEventLoopGroup(NioIoHandler.newFactory());
-				builder.transportSettings(TransportSettings.nettyBuilder().eventLoopGroup(this.eventLoopGroup).build());
+				EventLoopGroup eventLoopGroup = new MultiThreadIoEventLoopGroup(NioIoHandler.newFactory());
+				this.eventLoopGroup = eventLoopGroup;
+				builder.transportSettings(TransportSettings.nettyBuilder().eventLoopGroup(eventLoopGroup).build());
 			}
 		}