Default to empty hash when fetching gem installer options

I was seeing `TypeError: can't dup NilClass` when running `jbundle` because `Gem.post_install` was yielding a `gem_installer` whose `options` were `nil`. Later on, this `nil` was getting passed to `Jars::JarInstaller::Dependency` via `ruby_maven_install_options=`, which calls `dup` on the nil options. This pull request defaults the options to an empty hash.