Fix bundle outdated --strict crash

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

This command would crash when there are some dependencies not used under
the current platform.

What is your fix for the problem, implemented in this PR?

Check for nil at the proper place.

Fixes #4130.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[simi]

Nice, I wasn't sure this is intended. Since if I understand it now, outdated gems with currently unsupported platform would not be reported, but they were before 2.2.0, which (I'm not sure) could be breaking change.

[deivid-rodriguez]

Ha, good catch! I assumed they were ignored in the past too. Definitely a less useful behaviour, I'll have a look and see if I can fix it.

[deivid-rodriguez]

@simi I double checked this and bundler 2.1.4 has the same behaviour. That behaviour could even be considered a bug, because bundle update --lock updates the lockfile for all platforms. But since the behaviour is unchanged from 2.1.4, I'd ship this to fix the crash, and open a separate ticket to fix the behaviour.

[deivid-rodriguez]

Thanks @simi!! 🎉