Locator dependency being created with invalid version value if it is not specified

[timocov]

Given the following configuration the plugin adds a dependency which dep.getVersion() returns "null" (a string with null value in it):

protobuf {
  protoc {
    artifact = 'com.google.protobuf:protoc'
  }
}

Before 0.9.6 it was correctly set to null. The issue introduced in #786.

Whilst everything it seems working fine, it doesn't feel like the correct behaviour and some plugins that operate with dependencies might break/behave incorrectly because of that.

[jjohannes]

This is a performance regression with Gradle repeatetly trying to download non-existing metadata files for the version "null". See more explanation in #802.