fix-event-decoder-vtt

[tabmatfournier]

• two null checks were missed when converting old records that can have null values for valid through timestamps

│       Trace:  java.lang.NullPointerException: Cannot invoke "java.lang.Long.longValue()" because the return value of "io.tabular.iceberg.connect.events.CommitTablePayload.vtts()" is null                        │
│               at io.tabular.iceberg.connect.channel.EventDecoder.convertPayload(EventDecoder.java:119)                                                                                                            │
│               at io.tabular.iceberg.connect.channel.EventDecoder.convertLegacy(EventDecoder.java:82)                                                                                                              │
│               at io.tabular.iceberg.connect.channel.EventDecoder.decode(EventDecoder.java:77)                                                                                                                     │
│               at io.tabular.iceberg.connect.channel.Channel.lambda$consumeAvailable$2(Channel.java:131)                                                                                                           │
│               at java.base/java.lang.Iterable.forEach(Iterable.java:75)                                                                                                                                           │
│               at io.tabular.iceberg.connect.channel.Channel.consumeAvailable(Channel.java:125)