Handle invocation with no available verbs or env vars

[cottsay]

It might be useful when measuring performance to invoke colcon with an entire class of extensions blocked. This change "handles" the case where no verbs are available and minimizes the output when there are no verbs or environment variables available.

When invoked normally, this change should not modify any existing behavior.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.39%. Comparing base (c7d8194) to head (83a5a76).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #620 +/- ## ========================================== + Coverage 83.37% 83.39% +0.02% ========================================== Files 66 66 Lines 3789 3794 +5 Branches 737 739 +2 ========================================== + Hits 3159 3164 +5 Misses 556 556 Partials 74 74 ```

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