max of NA returns 0

[brentp]

given, t.bed.gz with:

chrM	10819	10830	NA

and a conf of:

[[annotation]]
file="t.bed.gz"
columns = [4, 4]
ops=["max", "self"]
names=["max4", "self4"]

and a query of:

chrM	10820	.	G	A	89403.6	PASS	AC=2;AF=1;AN=2

vcfanno adds:

max4=0;self4=NA

to the info. for this case, for max4, it should either not output a value or it should be nan.