Be graceful if badge has no categories.

Fixes #181

`getSimilar` was assuming there would always be at least one STEAM category associated with a badge. That was not a good assumption.

Note: this was actually fixed earlier, by 7ef55c228165fc8e02b45314c3a2a4d6fd5cc24f, but the "fix" still passed back an error. This is more graceful in that it detects if there are no categories early and calls back with an empty set.