portage: Successful exit if sync=yes and no package provided

When you call the module with parameter sync=yes and without the package, it was always failed:
```
$ ansible -i stage 192.168.1.213 -m portage -a 'sync=yes'
192.168.1.213 | FAILED >> {
    "failed": true, 
    "msg": "", 
    "parsed": false
}
```
after changes:
```
$ ansible -i stage 192.168.1.213 -m portage -a 'sync=yes'
192.168.1.213 | success >> {
    "changed": false, 
    "msg": "Sync successfully finished."
}
```
And call with package parameter:
```
$ ansible -i stage 192.168.1.213 -m portage -a 'package=app-misc/screen sync=yes verbose=yes'
192.168.1.213 | success >> {
    "changed": true, 
    "cmd": [
        "/usr/bin/emerge", 
        "--verbose", 
        "app-misc/screen"
    ], 
    "msg": "Packages installed.", 
```