1151925: Improved filtered listing output when results are empty.

Fixed a bug/oversight in the --list-enabled and --list-disabled options in
the repos command which would print the "available repositories" header
without any repos in cases where all of the available repos have been
filtered out. A proper message will now be displayed instead.
To cut back on some of the indention, the if here could be flipped
```
   if not repos:
        print _("There were no available repositories matching the specified criteria.")
        print("+----------------------------------------------------------+")
```

The
```
for repo in repos:
    ...
```
could bounce out an indent as well I think, the 'for blah in blahs' will just do nothing on empty list.

Looks like the out 'if len(repos)' could be flipped too.

Also, afaict, repos is a list? The "len(repos)" shouldn't be neccasary. 

```
if len(repos) > 0:
```
  and
```
if len(repos):
```
 is also odd, and both could become
```
if repos:
```
 (or the negated version as mentioned above)