add-refs.xsl must not fail if object not found

See #3572, release log:

• cbef1d8 by @yegor256: comment off
• 9a99ae9 by @yegor256: remove another checker
• 1f6e8f4 by @yegor256: Merge pull request #3567 from ...
• c383964 by @yegor256: Merge pull request #3568 from ...
• e2eccea by @yegor256: remove checker
• 4c8b86b by @yegor256: shorter
• 1d0f346 by @yegor256: lints 11
• 9f34a5d by @yegor256: stricter "check" in XSD
• 239b15f by @yegor256: locator in XSD
• f900014 by @yegor256: stricter attribute-name format...
• 625a7b1 by @yegor256: stricter formatting in XSD
• b7f78d7 by @yegor256: stricter sheet formatting in X...
• 30ed93f by @yegor256: labeler
• a2d395a by @renovate[bot]: chore(deps): update jamesives/...
• bd9f4a9 by @renovate[bot]: chore(deps): update dependency...
• 70ad582 by @yegor256: Merge pull request #3566 from ...
• 1284d81 by @yegor256: Merge pull request #3565 from ...
• 899b891 by @yegor256: #xslqual: exclude
• 0a9cebf by @yegor256: #xslqual: exclude codacy
• 8c5e3a7 by @yegor256: #xslqual: reverr
• and 61 more...

Released by Rultor 2.0-SNAPSHOT, see build log