Hide verb block from help if there are no verbs

[cottsay]

Rather than show an empty verb block when there are no verbs, just hide the block from the help text entirely.

Before:

$ COLCON_EXTENSION_BLOCKLIST=colcon_core.verb colcon --help
usage: colcon [-h] [--log-base LOG_BASE] [--log-level LOG_LEVEL] {} ...

options:
  -h, --help            show this help message and exit
  --log-base LOG_BASE   The base path for all log directories (default: ./log, to disable: /dev/null)
  --log-level LOG_LEVEL
                        Set log level for the console output, either by numeric or string value (default: warning)

colcon verbs:
  {}

Environment variables:
  ...

After:

$ COLCON_EXTENSION_BLOCKLIST=colcon_core.verb colcon --help
usage: colcon [-h] [--log-base LOG_BASE] [--log-level LOG_LEVEL]

options:
  -h, --help            show this help message and exit
  --log-base LOG_BASE   The base path for all log directories (default: ./log, to disable: /dev/null)
  --log-level LOG_LEVEL
                        Set log level for the console output, either by numeric or string value (default: warning)

Environment variables:
  ...

Follow-up to #620

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.26%. Comparing base (aa80ae5) to head (9da103d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #654   +/-   ##
=======================================
  Coverage   87.26%   87.26%           
=======================================
  Files          67       67           
  Lines        3904     3904           
  Branches      773      773           
=======================================
  Hits         3407     3407           
  Misses        391      391           
  Partials      106      106           

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