Fix null reference in toggle entry attachments shortcut

[sigsignv]

• I have tested my changes
• I read this document: https://miniflux.app/faq.html#pull-request

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document.querySelector() possibly return null.

If entry has not attachment, return value is null. null.toggleAttribute('open') cause TypeError.

Optional chaining operator can fix it, supported by most modern browsers:

• Chrome: 80+ (2020-)
• Saferi: 13.4+ (2020-)
• Firefox: 74+ (2020-)

[sigsignv]

Sorry. I overlooked jshintrc.

[fguillot]

Using a if statement instead of an optional chaining operator should be pretty straightforward.