GitalyClient::ConflictsService#conflicts? should return true for conflicts with missing side

Conversation: gitaly#889 (closed)

Fixes cases like gitaly#949 (comment 55786024).

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• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
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  • Has been reviewed by UX
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  • Has been reviewed by Backend
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• Conform by the merge request performance guides
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• Squashed related commits together
• Internationalization required/considered

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