(maint) Use null value with Puppet 7

Previously, in using the unit test workflows with Puppet modules, we set
Puppet 7's PUPPET_FORGE_TOKEN environment variable to '' and in module
Gemfiles evaluated `!ENV['PUPPET_FORGE_TOKEN'].to_s.empty?`.

However, the latest PDK template's (3.5.1) Gemfile evaluates the
PUPPET_FORGE_TOKEN environment variable slightly differently, opting
instead to check whether it exists at all:

https://github.com/puppetlabs/pdk-templates/blob/3.5.1.2/moduleroot/Gemfile.erb#L63

Ruby confusingly evaluates everything besides false and nil as "truthy,"
so an empty string (like the one we set for PUPPET_FORGE_TOKEN) is
evaluated as true.

This commit updates Puppet 7's environment variable to be set to null
instead of an empty string to allow the workflow to be used with PDK
template 3.5.1

Author: mhashizume

.github/workflows/unit_tests_with_released_puppet_gem.yaml

@@ -17,7 +17,7 @@ jobs:
         include:
           - puppet_version: '7'
             ruby: '2.7'
-            puppet_forge_token: ''
+            puppet_forge_token: ${{ null }}
           - puppet_version: '8'
             ruby: '3.1'
             puppet_forge_token: 'YES'