Fix nil exception when no zones are returned for a region

[alexpop]

When running the cis-gcp-benchmark-level2 profile against my gcp project, I noticed errors like this for a few controls:

I made two changes to the PR, one to replace false with []. Otherwise controls like these:

google_compute_region(project: gcp_project_id, name: region_name).zone_names.each do |zone_name|

will raise an error as well since they expect zone_names to be array and not boolean.

Second change is on the nil checking of @region.zones. It was not returning [], here's my pry play:

[1] pry(#<#<Class:0x00007f9d7f9b7bb0>>)> @region.zones
=> nil
[2] pry(#<#<Class:0x00007f9d7f9b7bb0>>)> defined?(@region.zones)
=> "method"
[3] pry(#<#<Class:0x00007f9d7f9b7bb0>>)> puts "bingo" if !defined?(@region.zones)
=> nil
[4] pry(#<#<Class:0x00007f9d7f9b7bb0>>)> myvar=nil
=> nil
[5] pry(#<#<Class:0x00007f9d7f9b7bb0>>)> defined?(myvar)
=> "local-variable"
[6] pry(#<#<Class:0x00007f9d7f9b7bb0>>)> defined?(myvar2)
=> nil
[7] pry(#<#<Class:0x00007f9d7f9b7bb0>>)> puts "bingo" if !defined?(myvar)
=> nil
[8] pry(#<#<Class:0x00007f9d7f9b7bb0>>)> puts "bingo" if !defined?(myvar2)
bingo
=> nil

[skpaterson]

Thanks @alexpop 👍

[skpaterson]

Looks like rubocop is complaining about the below:

libraries/google_compute_region.rb:29:34: C: Style/NilComparison: Prefer the use of the nil? predicate.