Running with no args prints error

[tomsaleeba]

Steps:

1. use fast-nvm-fish 2a5ad55
2. run nvm (with no args)

Expected:
The help message for nvm is shown.

Actual:
The warning from this script is shown (not the issue here) and an error message from the test command:

nvm-fast: at least one argument is required
test: Missing argument at index 2

I think if you show that warning message, the script should halt because continuing without something the script states it needs is heading for failure.

Alternatively, don't worry about printing a warning and instead, set the default arg to --help and let the script continue. I'm a fan of this approach.