Throw exceptions on bad parameters or broken internal state rather than return null or neutral value.

### Apache NetBeans version

Apache NetBeans 18

### What happened

NBLS commands tend to return some neutral / empty value instead of throwing an exception when the arguments are bad, missing or malformed.This may hide incorrect client usage. It is better to return an exceptional CompletableFuture so the client does know. See comments in #6079 for example.

### How to reproduce

_No response_

### Did this work correctly in an earlier version?

No / Don't know

### Operating System

any

### JDK

any

### Apache NetBeans packaging

Apache VSNetBeans for VSCode

### Anything else

_No response_

### Are you willing to submit a pull request?

Yes

Start with the NBLS command implementations and the example discussed in issue #6079. Inventory commands that receive bad, missing, or malformed arguments or encounter broken internal state, then define coverage for exceptional CompletableFuture results; done means those cases no longer return neutral values.

Written by the indexing model from the issue text.