[rubygems/rubygems] Fix `gem install <non-existent-gem> --force` crash

Before:

```
$ gem install sfdsfdsfsdide --force
ERROR:  While executing gem ... (NoMethodError)
    undefined method `spec' for nil:NilClass

    @always_install << newest.spec
                             ^^^^^
```

After:

```
$ gem install sfdsfdsfsdide --force
ERROR:  Could not find a valid gem 'sfdsfdsfsdide' (>= 0) in any repository
```

https://github.com/rubygems/rubygems/commit/4e2bfd1101

1 files changed, 16 insertions, 0 deletions

diff --git a/test/rubygems/test_gem_commands_install_command.rb b/test/rubygems/test_gem_commands_install_command.rb
index 0365b2c408..47a97dae4b 100644
--- a/test/rubygems/test_gem_commands_install_command.rb
+++ b/test/rubygems/test_gem_commands_install_command.rb
@@ -277,6 +277,22 @@ ERROR:  Could not find a valid gem 'bar' (= 0.5) (required by 'foo' (>= 0)) in a
     assert_match(/ould not find a valid gem 'nonexistent'/, @ui.error)
   end
 
+  def test_execute_nonexistent_force
+    spec_fetcher
+
+    @cmd.options[:args] = %w[nonexistent]
+    @cmd.options[:force] = true
+
+    use_ui @ui do
+      e = assert_raise Gem::MockGemUi::TermError do
+        @cmd.execute
+      end
+      assert_equal 2, e.exit_code
+    end
+
+    assert_match(/ould not find a valid gem 'nonexistent'/, @ui.error)
+  end
+
   def test_execute_dependency_nonexistent
     spec_fetcher do |fetcher|
       fetcher.spec 'foo', 2, 'bar' => '0.5'