[BUGFIX] Fix behaviour of strictly allowed RTE classes
[BUGFIX] Fix behaviour of strictly allowed RTE classes

The list of allowed classes missed the "empty" element, which auto-fixed
not-allowed classes to get the first class assigned automatically.

Example:

```
RTE.default.proc.entryHTMLparser_db {
    p.fixAttrib.class.list = lead, text-left, text-center
}
```

Source:

```
<p class="cool-legacy-class">..Bla..</p>
```

Will get cleaned up to

```
<p class="lead">..Bla..</p>
```

Adding the empty list-element fixes this faulty behaviour:

```
RTE.default.proc.entryHTMLparser_db {
    p.fixAttrib.class.list = , lead, text-left, text-center
}
```

MediaCluster/bootstrap_package,benjaminkott/bootstrap_package,stweil/bootstrap_package,maddy2101/bootstrap_package,stweil/bootstrap_package,benjaminkott/bootstrap_package,cedricziel/bootstrap_package-1,webian/bootstrap_package,benjaminkott/bootstrap_package,s-leger/bootstrap_package,MediaCluster/bootstrap_package,stweil/bootstrap_package,maddy2101/bootstrap_package,s-leger/bootstrap_package,s-leger/bootstrap_package,cedricziel/bootstrap_package-1,webian/bootstrap_package,maddy2101/bootstrap_package,webian/bootstrap_package,stweil/bootstrap_package,MediaCluster/bootstrap_package,benjaminkott/bootstrap_package,cedricziel/bootstrap_package-1