`alt_alleles` dimension is not always being added for Number=A fields

[tomwhite]

Found while working on sgkit-dev/vcztools#198

Note that the Number=A becomes Number=1 in the following conversion:

$ bcftools view --header-only tests/data/vcf/1kg_2020_chr20_annotations.bcf | grep MLEAF
##INFO=<ID=MLEAF,Number=A,Type=Float,Description="Maximum likelihood expectation (MLE) for the allele frequency (not necessarily the same as the AF), for each ALT allele, in the same order as listed">
$ vcf2zarr convert --force tests/data/vcf/1kg_2020_chr20_annotations.bcf 1kg_2020_chr20_annotations.vcz
$ vcztools view --header-only 1kg_2020_chr20_annotations.vcz | grep MLEAF
##INFO=<ID=MLEAF,Number=1,Type=Float,Description="Maximum likelihood expectation (MLE) for the allele frequency (not necessarily the same as the AF), for each ALT allele, in the same order as listed">