Keep namespace relative if base_prefix is empty string

[slivingston]

Suppose that rather than use base_prefix to change the default of "mobile_base", a user wishes to entirely remove the base prefix. If the param base_prefix is set to the empty string, then the topic names would become absolute, i.e., would begin with "/". This changeset ensures that they remain relative in this case.

The motivating use-case is spawning multiple instances of the Kobuki model, each with a unique namespace, while trying to keep topic names short.