Fixed failing self-update (bsc#1258142)

[lslezak]

Problem

• https://bugzilla.suse.com/show_bug.cgi?id=1258142
• The self-update repository is not ready yet and the self-update process fails.

Solution

• If the SCC returns empty list then do not try using the fallback URL which is the same as expected response from SCC.
• Empty list from the SCC means the self-update repository does not exist yet so skip the self-update process.
• Use the self-update fallback URL only when querying SCC fails or there is a network error.

Testing

• Tested manually

The self-update is skipped and no error is reported.

[coveralls]

coverage: 75.041%. remained the same
when pulling 880be28 on self-update-fallback
into 8ba481c on master.