enhance client side for API name check
enhance client side for API name check

the first argument to sdkclient.call is target API name, it should
be a string and not null. If not, don't waste time to send data to
server, just return error to the caller.

Change-Id: Ifc8ab76c55c5b56b7a7d8e09e0e39a83636e4f7e

mfcloud/python-zvm-sdk,mfcloud/python-zvm-sdk,mfcloud/python-zvm-sdk,mfcloud/python-zvm-sdk