[7.x] fix: avoid panic when sourcemap doc does not contain hit (#4619)

[simitt]

Backports the following commits to 7.x:

• fix: avoid panic when sourcemap doc does not contain hit (fix: avoid panic when sourcemap doc does not contain hit #4619)

[apmmachine]

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Build stats

• Build Cause: Pull request #4624 opened

• • Start Time: 2021-01-14T13:37:47.086+0000

• Duration: 47 min 7 sec

• Commit: 57fd91b

Test stats 🧪

Test	Results
Failed	0
Passed	4513
Skipped	126
Total	4639

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• Description: tar --exclude=coverage-files.tgz -czf coverage-files.tgz coverage

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• Description: tar --exclude=system-tests-linux-files.tgz -czf system-tests-linux-files.tgz system-tests