Graylog 3.1 CEF UDP regression

Since migrate to Graylog 3.1, the CEF UDP input was unable to decode message (works fine in 2.4).

## Expected Behavior
Decode CEF Message from Checkpoint Firewall

`
CEF:0|Check Point|VPN-1 & FireWall-1|Check Point|Log|sqlnet1|Unknown|act=Drop deviceDirection=0 rt=1575387805000 spt=51690 dpt=1521 cs2Label=Rule Name layer_name=Prod_fw Security layer_uuid=927f269b-52dd-4cab-8550-7aeffda6ca02 match_id=4019 parent_rule=0 rule_action=Drop rule_uid=3e517596-1229-c34a-86e4-ab3a7f3ff752 ifname=bond1.45 logid=0 loguid={0x0,0x0,0x0,0x0} origin=10.254.4.203 originsicname=CN=fw,O=fw.net.m9ug26 sequencenum=292 version=5 dst=10.140.27.121 inzone=Internal outzone=Internal product=VPN-1 & FireWall-1 proto=6 service_id=sqlnet1 src=10.241.45.1
`

## Current Behavior
Error in decode process

`2019-12-03T16:52:09.226+01:00 ERROR [DecodingProcessor] Error processing message RawMessage{id=dcf8ef90-15e4-11ea-b6a8-0050568a1266, journalOffset=-9223372036854775808, codec=CEF, payloadSize=19, timestamp=2019-12-03T15:52:09.225Z, remoteAddress=/10.145.24.247:38344}
java.lang.NullPointerException: null
at org.graylog.plugins.cef.parser.MappedMessage.(MappedMessage.java:37) ~[graylog.jar:?]
at org.graylog.plugins.cef.codec.CEFCodec.decodeCEF(CEFCodec.java:128) ~[graylog.jar:?]
at org.graylog.plugins.cef.codec.CEFCodec.decode(CEFCodec.java:117) ~[graylog.jar:?]
at org.graylog2.shared.buffers.processors.DecodingProcessor.processMessage(DecodingProcessor.java:150) ~[graylog.jar:?]
at org.graylog2.shared.buffers.processors.DecodingProcessor.onEvent(DecodingProcessor.java:91) [graylog.jar:?]
at org.graylog2.shared.buffers.processors.ProcessBufferProcessor.onEvent(ProcessBufferProcessor.java:86) [graylog.jar:?]
at org.graylog2.shared.buffers.processors.ProcessBufferProcessor.onEvent(ProcessBufferProcessor.java:45) [graylog.jar:?]
at com.lmax.disruptor.WorkProcessor.run(WorkProcessor.java:143) [graylog.jar:?]
at com.codahale.metrics.InstrumentedThreadFactory$InstrumentedRunnable.run(InstrumentedThreadFactory.java:66) [graylog.jar:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_222]`

## Steps to Reproduce (for bugs)

1.Add CEF input
2.send CEF message from Checkpoint Firewall log manager
3.Error in Graylog log (in 2.4 no error)

## Context

## Your Environment

* Graylog Version: 3.1.3
* Elasticsearch Version: 6.8.4
* MongoDB Version: 2.6.12-6.el7
* Operating System: CentOS 7.7
* Browser version: Chrome 71.0.3578.98