Use empty array when spec_settings returns nil

If use bundler 1.14.4 with rubygems 2.6.8 and
`Bundler.settings["build.#{spec.name}"]` returns `nil` then we get
error "can't modify frozen literal string" from rubygems[1].
RubyGems 2.6.8 is the default version for Ruby2.4.0.

[1]: https://github.com/rubygems/rubygems/blob/v2.6.8/lib/rubygems/ext/rake_builder.rb#L13

See also ku1ik/rainbow#48

lib/bundler/installer/gem_installer.rb

@@ -52,7 +52,7 @@ def spec_settings
     end
 
     def install
-      spec.source.install(spec, :force => force, :ensure_builtin_gems_cached => standalone, :build_args => [spec_settings])
+      spec.source.install(spec, :force => force, :ensure_builtin_gems_cached => standalone, :build_args => Array(spec_settings))
     end
 
     def install_with_settings