(fix) handle nil repos in github responses

[tphoney]

No description provided.

[bradrydzewski]

another option could be to move the nil check into the convert list function, and prevent nil repositories from being appended to the list all together

// helper function to convert from the gogs repository list to
// the common repository structure.
func convertRepositoryList(from []*repository) []*scm.Repository {
	to := []*scm.Repository{}
	for _, v := range from {
+		if v != nil {
			to = append(to, convertRepository(v))
+		}
	}
	return to
}

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