[receiver/sqlquery] fix handling of nulls

[pmcollins]

Description: This fixes the receiver panicking on NULL values. Additional refactoring was done to improve test coverage.

Link to tracking Issue: #19177

Testing: Unit tests, integration tests, and manual tests passed.

Documentation: Docs added about NULL handling.

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Foresight Summary

    
Major Impacts

build-and-test-windows duration(6 seconds) has decreased 39 minutes 55 seconds compared to main branch avg(40 minutes 1 second).

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⭕  build-and-test-windows workflow has finished in 6 seconds (39 minutes 55 seconds less than main branch avg.) and finished at 17th Mar, 2023.

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✅  telemetrygen workflow has finished in 1 minute 4 seconds (41 seconds less than main branch avg.) and finished at 17th Mar, 2023.

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✅  check-links workflow has finished in 1 minute 44 seconds and finished at 17th Mar, 2023.

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✅  changelog workflow has finished in 2 minutes 14 seconds and finished at 17th Mar, 2023.

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✅  build-and-test workflow has finished in 35 minutes 13 seconds (19 minutes 13 seconds less than main branch avg.) and finished at 17th Mar, 2023.

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unittest-matrix (1.20, connector)	-     🔗	✅ 126  ❌ 0  ⏭ 0    🔗	See Details
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✅  prometheus-compliance-tests workflow has finished in 3 minutes 19 seconds (3 minutes 54 seconds less than main branch avg.) and finished at 17th Mar, 2023.

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Job	Failed Steps	Tests
prometheus-compliance-tests	-     🔗	✅ 21  ❌ 0  ⏭ 0    🔗	See Details

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✅  e2e-tests workflow has finished in 11 minutes 21 seconds (3 minutes 49 seconds less than main branch avg.) and finished at 17th Mar, 2023.

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✅  load-tests workflow has finished in 15 minutes 2 seconds (⚠️ 2 minutes 36 seconds more than main branch avg.) and finished at 17th Mar, 2023.

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