[Small] Better handling for global commands without arguments

[charlespierce]

Info

• Currently, if a user runs a global install or uninstall without any package arguments (e.g. npm i -g or yarn global remove), we show:

note: Volta is processing each package separately

• However, since there are no arguments, we then effectively no-op by iterating over an empty list.
• This leads to a confusing situation where there's no feedback about success or failure to the user (and the return code is 0, indicating success).
• Since, in these situations, there aren't any tools for Volta to handle, we should treat it like a non-global situation and allow the underlying package manager to show whatever error it normally would when the user forgets the arguments.

Changes

• Updated the parser for both npm and Yarn to handle the case of no package arguments and treat it like a non-global command.

Tested

• Added new tests to cover this use case, confirming that the note text is not shown.