Build stops if no k-mers in minimiser file

When using `--kmer-count-cutoff > 1` some minimisers files were empty for user bins with only unique kmers:

`[Error] The file raptor4/out/3060649.minimiser is empty`

Empty user bins should be skipped in the build process and a warning issued to the user informing this. Maybe this can be already done in the layout phase.
