wsk activation get <id> --last is buggy

This test is a false positive, and will need a unit go test to properly test this:

https://github.com/apache/incubator-openwhisk-cli/blob/ed795ede797e3730669cdaa2453be96e5f5e7d0b/tests/src/test/scala/system/basic/WskCliBasicTests.scala#L973

The test succeeds only because there are activations in the database. Try it on an empty database:

```
> wsk -i activation get x --last
error: Unable to get activation: Activation list does not contain any activations.
Run 'wsk --help' for usage.
```

the parameter validation is wrong and it goes on to query the API for the activations, which should not happen.

Start with tests/src/test/scala/system/basic/WskCliBasicTests.scala at line 973 and reproduce the `wsk activation get --last` case against an empty database. Trace the command's parameter validation and add the requested unit Go test. Done means the invalid case is caught before querying the API and the test no longer passes only when activations exist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.