fix expansion for the case of ENV var is not set

[pvshewale]

This is a 🐞 bug fix.

• I've added tests (if it's a bug, feature or enhancement)
• I've adjusted the documentation (if it's a feature or enhancement)
• The test suite passes (run bundle exec rspec to verify this)

Summary

As per docs, with Terraspace v2, any environment variable is also available in the expansion helper. IE: MY_VAR=foo can be use in the expansion(":MY_VAR") method.
This works as expected if MY_VAR is set to some value, but in case environment variable MY_VAR is not set it's returning as it is in non-expanded state like ":MY_VAR"
Now with this fix, it returns empty space if environment variable is not set. Unit test case is also updated.

Context

How to Test

Just run the unit test

rspec spec/terraspace/provider/expander/generic_spec.rb

Version Changes

[tongueroo]

Thanks 👍 Released in 2.2.4

[pvshewale]

Thanks 👍