Fix show_envvar output for a sequence or an empty envvar

[kdeldycke]

This PR fixes some awkward rendering in the Error: Invalid value for '--units' (env var: xxxxx) error message's hint for special cases of environment variables:

Parameter value	Hint before	Hint after
envvar=None	Nothing	Nothing
envvar=""	(env var: '')	Nothing
envvar=[]	(env var: '[]')	Nothing
envvar=["WEATHER_UNITS", "UNITS"]	(env var: '['WEATHER_UNITS', 'UNITS']')	(env var: 'WEATHER_UNITS', 'UNITS')

Also fixes the rendering of help screen:

Parameter value	Help screen before	Help screen after
envvar=None	Nothing	Nothing
envvar=""	[env var: ]	Nothing
envvar=[]	[env var: ]	Nothing