NewProvidesAllArch: Check if group is empty before using it

APT 1.9.6 introduced empty groups by making use of groups to
deduplicate package names. This is not normally a problem, but
here we assumed that every group has at least one package.

This caused a problem because automake was providing automake-1.16
while having the source package automake-1.16. So we found the
automake-1.16 group, iterated over its empty package list, trying
to store the provides (which hence never happened).

LP: #1859952

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