(FACT-3186) Handle partitions fact when none exist

[mhashizume]

Prior to this commit, when a diskless machine used Facter the partitions fact errored out with an undefined method error. This was because the machine did not have any partitions and Facter was unable to dig into the partitions hash because the hash didn't exist.

This commit updates Linux's partition fact to return nil early in the partitions method if the partitions resolver returns nothing.

[mhashizume]

Opening as a draft PR until I can do more testing

[joshcooper]

Wondering if these other facts have the same issue:

❯ git grep -A2 ' mountpoints\.each ' lib
lib/facter/facts/freebsd/mountpoints.rb:        mountpoints.each do |mnt|
lib/facter/facts/freebsd/mountpoints.rb-          fact[mnt[:path].to_sym] = mnt.reject { |k| k == :path }
lib/facter/facts/freebsd/mountpoints.rb-        end
--
lib/facter/facts/linux/mountpoints.rb:        mountpoints.each do |mnt|
lib/facter/facts/linux/mountpoints.rb-          fact[mnt[:path].to_sym] = mnt.reject { |k| k == :path }
lib/facter/facts/linux/mountpoints.rb-        end
--
lib/facter/facts/solaris/mountpoints.rb:        mountpoints.each do |mnt|
lib/facter/facts/solaris/mountpoints.rb-          fact[mnt[:path].to_sym] = mnt.reject { |k| k == :path }
lib/facter/facts/solaris/mountpoints.rb-        end

[mhashizume]

@joshcooper I don't think we need to worry about those facts because this should only be an issue with partitions, right?

[mhashizume]

After some feedback from the user who reported this issue, I've updated my fix

[mhashizume]

Confirmed with the user in the ticket that this fixes the issue.