Fix bundlebuilder not handling empty arguments
Fix bundlebuilder not handling empty arguments

If no arguments are passed, parser.parse_args() returns
command = None.

Now checks if function exists in globals() before executing.

Traceback:
```
$ python2 ./setup.py
usage: ./setup.py [-h]
                  {install,check,dist_xo,dist_source,build,fix_manifest,genpot,dev}
                  ...
./setup.py: error: too few arguments

$ python3 ./setup.py
Traceback (most recent call last):
  File "./setup.py", line 5, in <module>
    bundlebuilder.start()
  File "/usr/lib/python3.7/dist-packages/sugar3/activity/bundlebuilder.py", line 637, in start
    globals()['cmd_' + options.command](config, options)
TypeError: can only concatenate str (not "NoneType") to str
```

sugarlabs/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3