Query not formatting missing float fields correctly

[tomwhite]

They are showing as nan rather than .:

% vcztools query -f '%AF{0}\n' vcz_test_cache/sample.vcf.vcz
nan
nan
0.5
0.017
0.333
nan
nan
nan
nan

Compare to:

% bcftools query -f '%AF{0}' tests/data/vcf/sample.vcf.gz    
.
.
0.5
0.017
0.333
.
.
.
.