Make Sqrl std library functions robust to null input

[mbroecheler]

When epochtotimestamp gets a null value, it throws an exception. It should return null instead:

java.lang.NullPointerException: null
        at com.datasqrl.time.AbstractEpochToTimestamp.eval(AbstractEpochToTimestamp.java:20) ~[sqrl-test.jar:0.5.6]
        at StreamExecCalc$7.processElement(Unknown Source) ~[?:?]
        at org.apache.flink.streaming.runtime.tasks.CopyingChainingOutput.pushToOperator(CopyingChainingOutput.java:75) ~[sqrl-test.jar:0.5.6]
        at org.apache.flink.streaming.runtime.tasks.CopyingChainingOutput.collect(CopyingChainingOutput.java:50) ~[sqrl-test.jar:0.5.6]
        at org.apache.flink.streaming.runtime.tasks.ChainingOutput.collectAndCheckIfChained(ChainingOutput.java:90) ~[sqrl-test.jar:0.5.6]
        at org.apache.flink.streaming.runtime.tasks.ChainingOutput.collectAndCheckIfChained(ChainingOutput.java:40) ~[sqrl-test.jar:0.5.6]
        at org.apache.flink.streaming.runtime.tasks.BroadcastingOutputCollector.collect(BroadcastingOutputCollector.java:83) ~[sqrl-test.jar:0.5.6]
        at org.apache.flink.streaming.runtime.tasks.BroadcastingOutputCollector.collect(BroadcastingOutputCollector.java:34) ~[sqrl-test.jar:0.5.6]
        at

See AbstractEpochToTimestamp and also double check the other timestamp functions in StdTimeLibraryImpl