Do not interpolate empty prefix

[brunocodutra]

Imported from jantimon/favicons-webpack-plugin#110

An empty prefix setting (i.e.: install all icons into the document root)
was treated by loaderUtils.interpolateName() as '[hash].[ext]'.
While this may be a useful default in other contexts, it is wrong here
where a directory template gets interpolated. So handle the empty string
specially.