Fix to avoid polluting $error on missing modules.

When a completer has a script block to determine
the commands to register for, the script block
might not find any commands, in which case we'd
try to register a completer with $null for the
command name and that's not allowed.

Fix is to skip registering anything in this case.
Merge branch 'master' of https://github.com/lzybkr/TabExpansionPlusPlus
Undo the change to check if $parameters is missing a CommandName.

Jason's change fixes it by not putting such items into the background results queue in the first place.