LibJS: Only set element in array literal to an empty value if it's null
This avoids an unnecessary empty load that immediately gets overridden.
For example, `[1,,]` would appear as:
[ 0] EnterScope
[ 10] LoadImmediate value:<empty>
[ 28] LoadImmediate value:1
[ 40] Store dst:$1
[ 48] LoadImmediate value:<empty>
[ 60] Store dst:$2
[ 68] NewArray, elements:[$1,$2]
But now appears as:
[ 0] EnterScope
[ 10] LoadImmediate value:1
[ 28] Store dst:$1
[ 30] LoadImmediate value:<empty>
[ 48] Store dst:$2
[ 50] NewArray, elements:[$1,$2]