don't assume the Include attribute is present on a <ProjectReference> node

[brettfo]

When parsing project references, we were assuming all of the elements looked like this:

...
<ProjectReference Include="path\to\other.csproj" />
...

If that Include attribute isn't present, then we'll eventually throw with:

undefined method `tr' for nil:NilClass

          relative_path = ref_path.tr("\\", "/")
                                  ^^^

The fix is to simply skip processing that particular <ProjectReference> element if it doesn't have the requisite attribute. Similar behavior is also present elsewhere in project_file_parser.rb, so it's a known pattern that we just missed applying here.

Of all of the current undefined method '_' for nil:NilClass errors in NuGet, this is the largest class.