fixes #5142 - fixing error on repo create with empty string

pulp does not treet empty string for feed urls the same as
nil.  As a result repo creation would fail.  This fixes the issue by doing 2 things:

a) convert empty string to nil at repo creation time
b) explicitly do not accept empty string in the url validator
Merge pull request #3969 from jlsherrill/repo_feed

fixes #5142 - fixing error on repo create with empty string