Don't fail with NPE when there is no debug variant

I assumed there is always at least one debug variant since it is automatically created by the android plugin. But in fact it is possible for a user to disable the debug variant using such code:

android.variantFilter { variant ->
  if(variant.buildType.name.equals('debug')) {
    variant.setIgnore(true);
  }
}

When there is no debug variant, we should fallback to use the first variant, whatever is the build type.

The workaround is to manually specify the variant to be used:

sonarqube {
  androidVariant = "xxxxx"
}