Aggregate function errors

[brynrhodes]

The following functions return incorrect results:

define "AllTrueIsTrueWhenNull": AllTrue(null) // Should be true, returns null
define "AnyTrueIsFalseWhenNull": AnyTrue(null) // Should be false, returns null
define "StdDevIsNull": StdDev({ null as Quantity, null as Quantity, null as Quantity }) // Gives NullPointerException, should be null
define "PopulationStdDevIsNull": PopulationStdDev({ null as Quantity, null as Quantity, null as Quantity }) // Gives NullPointerException, should be null

Note that these are directly from the spec, and are likely in the cql-tests, so the fix here should be coordinated with a passing test in the tests suite.