Empty classifier in dependency causes extra dash

When having a dependency with an empty classifier element, adding it as a dependency set causes an extra dash to appear in the file name.
For example, when I add a dependency of this type (notice the empty classifier element):

[snip]
<dependency>
<groupId>org.apache.tiles</groupId>
<artifactId>tiles-api</artifactId>
<version>2.2.2</version>
<classifier />
</dependency>
[/snip]

and I put it inside the assembly descriptor:
[snip]
<dependencySets>
<dependencySet>
<outputDirectory>/</outputDirectory>
</dependencySet>
</dependencySets>
[/snip]

I see that the tiles-api jar contains an extra dash in its name:
tiles-api-2.2.2-.jar