For some reason `gem install` unsets the GEM_PATH environment variable
internally unless the install dir is provided. This in turn means that
if it invokes extconf.rb and extconf.rb depends on a gem available on
the GEM_PATH (like pkg-config for nokogiri) then it's not available in
that context.
Proof: d8293c4729/lib/rubygems/commands/install_command.rb (L151)
Blame: 9ea600c9c2
This is a hack that sets the :install_dir to where we would install
anyways (the GEM_HOME is the default installation destination).
lib.escapeShellArgs doesn't work well when a null value is provided.
[] is also the correct value since it's really just an empty list of
arguments that we have.
