A user reported that the `api-get` command was failing with an error attempting to run without explicitly listing the server name in the command-line arguments or `.reviewboardrc`. In this case, they were using the `reviewboard.url` perforce counter to define the server name for their repository. When there's no explicit server name configured, we will initialize the repository client(s) in order to use them to detect the server name. Unfortunately, the `api-get` command didn't include the repository command-line arguments (because why would it), and the code path that does this was assuming that it existed. This change fixes it so we only conditionally look for the value of the --repository-type argument inside the argparse results. If not found, we'll then proceed through our normal repository detection code. Testing Done: Set up a perforce server that defined the `reviewboard.url` counter, and then ran `rbt api-get users` from within a perforce client. Saw that it correctly determined my Review Board host to use, rather than crashing with an AttributeError.