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.