(Maint) Don't parse nil Facter::Util::Resolution output

Previously, running facter on a system without virt-what would produce:

  Could not retrieve virtual: undefined method `gsub' for nil:NilClass

This commit checks that we have a non-nil output before trying to parse
it.
(Maint) Suppress spec test output

The macosx_spec ensures that an error is raised when parsing invalid
XML, but this causes the following output when running the tests

  Fatal error: error parsing attribute name at :1.
  Fatal error: attributes construct error at :1.
  Fatal error: Couldn't find end of Start Tag bad line 1 at :1.
  Fatal error: Extra content at the end of the document at :1.

This commit suppresses the output written to STDERR. See
LibXML::XML::Error::VERBOSE_HANDLER.
(#20053) Add missing require in ipaddress6 fact

This fixes the error
    Could not retrieve ipaddress6: uninitialized constant Facter::Util::IP
(Maint) Fix environment leakage in loader spec

This removes the stub on Facter when loading facts from the environment. This
would cause the test to fail if the environment had any extra variables named
FACTER_*.
Merge branch '1.7.x'

* 1.7.x:
  (Maint) Fix environment leakage in loader spec
  (#20053) Add missing require in ipaddress6 fact
  (Maint) Suppress spec test output
  (Maint) Don't parse nil Facter::Util::Resolution output

Conflicts:
	spec/unit/util/loader_spec.rb