build: Restore Doxygen output for MediaWiki release tags (take 3)

Follows-up:
* I696f1e7ede (60f106e112): Apply `vendor` and `node_modules` exclude
  patterns even when `--file` is used to generate docs for an extension
  or a subset of core.

* I9313457796 (eaf491b4f4): Use faster and upstream supported EXCLUDE
  instead of EXCLUDE_PATTERNS for skipping whole top-level dirs.

Neither of these addressed the fact that this `if` condition is not
actually possible to reach because `explode()` always returns an
array with at least one item. e.g. `exclude(',', '') == [ '' ]`.

Test plan:
* `php maintenance/mwdocgen.php --output ./docs/ --version whatever`
* Review the contents the mentioned `/tmp/MWDocGen-*` file that is
  auto-generated.

Before:
```
INPUT                  =  ""
…
EXCLUDE                =  cache images
```

After:
```
INPUT                  =
…
EXCLUDE                =  cache images extensions skins
```

Bug: T317451
Change-Id: I871b321fa9fdb2d763fba532ab6248b0fc4b1701

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