(#10417) Disable diff on Windows by default

Previously, anyone running puppet apply or puppet agent with
either the --test, --noop, or --show_diffs option, would see a
CreateProcess exception while trying to execute the diff
command. This was made worse by the recent change that causes puppet
to diff its last run summary file.

On Windows, diff is not present, by default. There is fc.exe (file
compare), but it is rather brain-dead, especially if you diff a binary
file.

This commit makes Puppet::Util::Diff.diff return '' by default on
Windows. However, if a diff command has been specified in puppet.conf,
then puppet will use it, as before.