fix: don't throw when there are no commands

[gustavohenke]

There's a guard against no inputs in

concurrently/src/concurrently.ts

Line 170 in 4ffe63b

assert.notStrictEqual(baseCommands.length, 0, '[concurrently] no commands provided');

However, there can still a situation where are no commands to run if one of the inputs is a wildcard that doesn't expand to anything.
This has been detected as the cause of #527, though it's not a fix for it. CompletionListener should simply assume that no commands = success.

[coveralls]

coverage: 98.186% (+0.006%) from 98.18%
when pulling d599be5 on no-commands
into 4ffe63b on main.