[Flink-4130] CallGenerator could generate illegal code when taking no operands

[unsleepy22]

No description provided.

[wuchong]

It seems that when operands.size == 0 and nullCheck is enable, the $nullTerm will always be true, which means $resultTerm always be the default value. That may be wrong.

[unsleepy22]

@wuchong although I agree this bug could be fixed by adding operands.nonEmpty, there is a problem in generated code, specifically, an undeclared variable will be generated like this:

long result$16 = 
((org.apache.flink.api.java.table.TumblingWindowUDf)
udfDict.get("tumblingWin")).
  eval()
           ;

if (isNull$17) {
  out.setField(1, null);
}
else {
  out.setField(1, result$16);
}

I guess it's generated in GeneratedExpression, for simplicity I reverted my change.

[wuchong]

Yea... the isNull$17 is not declared if we do what I said above, and of course will throw compile error.

The code looks good to me now, although it looks a little weird that $nullTerm always be false.

[twalthr]

Thanks for the contribution @unsleepy22.
I changed the code a little bit, so that we can prevent an if(false) branch.

Will merge...