added default gradle.properties containing empty sonatypeUserName and sonatypePassword

Using this library via gradle, I had the following error when trying to build:

    no such property: sonatypeUserName

With this gradle.properties file, sonatypeUserName and sonatypePassword are set with empty values, avoiding the previous error.

If the user already has a sonatypeUserName set to deploy on maven in his **${GRADLE_HOME}/gradle.properties**, it already overrides the present file, so no changes are requested!