(bug) Return nil when GitHub and GitLab queries return non-200 code

This fixes a bug in the `GitHub` and `GitLab` spec searchers that
prevented the spec searcher from using the `GitLab` and `GClone` spec
searchers. Previously, if either spec searcher returned a non-200
response code, they would return an empty string. This prevented the
`Git` spec searcher from using any other spec searcher other than
`GitHub` to retrieve module metadata.

Author: beechtom

lib/puppetfile-resolver/spec_searchers/git/github.rb

@@ -43,7 +43,7 @@ def self.metadata(puppetfile_module, resolver_ui, config)
 
           if response.code != '200'
             resolver_ui.debug(err_msg + "Expected HTTP Code 200, but received #{response.code}")
-            return ''
+            return nil
           end
           response.body
         end

lib/puppetfile-resolver/spec_searchers/git/gitlab.rb

@@ -45,7 +45,7 @@ def self.metadata(puppetfile_module, resolver_ui, config)
 
           if response.code != '200'
             resolver_ui.debug(err_msg + "Expected HTTP Code 200, but received #{response.code}")
-            return ''
+            return nil
           end
           response.body
         end