Created catch in bootgen for no pops.

the function `truenames()` acts differently if there are no
populations.
fixed catch for missing treated as zeroes in bruvo.dist

Adegenet has three ways of treating missing data:

 1. leave it as `NA`
 2. convert it to `0`, representing mysterious extra alleles
 3. convert it to the mean of the observed data.

Since converting the data from allele frequencies to actual alleles
with missing data can be problematic, two catches were set up to weed
out the ambiguous cases. The first catch caught floats that were not
possible with the given ploidy. The second attempted to catch zeroes,
but ended up catching everything else. Now it will catch only zeroes
and NA’s can be treated normally.