[processor/transform] Fix panic when transforming span events

[TylerHelmuth]

Description:
Fixes a bug that would cause the collector to panic when transforming span events

Testing:
Added new unit test

[runforesight]

Foresight Summary

    
Major Impacts

build-and-test duration(43 minutes 48 seconds) has decreased 12 minutes 5 seconds compared to main branch avg(55 minutes 53 seconds).

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⭕  tracegen workflow has finished in 7 minutes 43 seconds (⚠️ 4 minutes 58 seconds more than main branch avg.) and finished at 5th Dec, 2022.

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Job	Failed Steps	Tests
build-dev	-     🔗	N/A	See Details
publish-latest	-     🔗	N/A	See Details
publish-stable	-     🔗	N/A	See Details

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⭕  check-links workflow has finished in 7 minutes 53 seconds (⚠️ 5 minutes 31 seconds more than main branch avg.) and finished at 5th Dec, 2022.

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Job	Failed Steps	Tests
changed files	-     🔗	N/A	See Details
check-links	-     🔗	N/A	See Details

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⭕  prometheus-compliance-tests workflow has finished in 7 minutes 56 seconds and finished at 5th Dec, 2022.

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Job	Failed Steps	Tests
prometheus-compliance-tests	-     🔗	N/A	See Details

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⭕  build-and-test-windows workflow has finished in 13 minutes 30 seconds (20 minutes 10 seconds less than main branch avg.) and finished at 5th Dec, 2022.

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Job	Failed Steps	Tests
windows-unittest-matrix	-     🔗	N/A	See Details
windows-unittest	-     🔗	N/A	See Details

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❌  changelog workflow has finished in 23 minutes 23 seconds (⚠️ 18 minutes 13 seconds more than main branch avg.) and finished at 5th Dec, 2022. 0 job failed.

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Job	Failed Steps	Tests
changelog	-     🔗	N/A	See Details

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✅  load-tests workflow has finished in 1 hour 14 minutes 16 seconds (⚠️ 59 minutes 14 seconds more than main branch avg.) and finished at 5th Dec, 2022.

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Job	Failed Steps	Tests
loadtest (TestTraceBallast1kSPSWithAttrs|TestTraceBallast1kSPSAddAttrs)	-     🔗	✅ 10  ❌ 0  ⏭ 0    🔗	See Details
loadtest (TestMetricResourceProcessor|TestTrace10kSPS)	-     🔗	✅ 12  ❌ 0  ⏭ 0    🔗	See Details
loadtest (TestTraceNoBackend10kSPS|TestTrace1kSPSWithAttrs)	-     🔗	✅ 8  ❌ 0  ⏭ 0    🔗	See Details
loadtest (TestBallastMemory|TestLog10kDPS)	-     🔗	✅ 19  ❌ 0  ⏭ 0    🔗	See Details
setup-environment	-     🔗	N/A	See Details
loadtest (TestIdleMode)	-     🔗	N/A	See Details
loadtest (TestMetric10kDPS|TestMetricsFromFile)	-     🔗	N/A	See Details
loadtest (TestTraceAttributesProcessor)	-     🔗	N/A	See Details

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 build-and-test workflow has finished in 1 hour 28 minutes 14 seconds (⚠️ 31 minutes 43 seconds more than main branch avg.) and finished at 5th Dec, 2022.

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Job	Failed Steps	Tests
unittest-matrix (1.19, internal)	N/A	✅ 592  ❌ 0  ⏭ 0    🔗	See Details
correctness-traces	N/A	✅ 0  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, processor)	N/A	✅ 1466  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, extension)	N/A	✅ 528  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, processor)	N/A	✅ 1466  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, receiver-0)	N/A	✅ 2532  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, exporter)	N/A	✅ 2412  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, exporter)	N/A	✅ 2412  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, receiver-0)	N/A	✅ 2532  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, internal)	N/A	✅ 592  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, other)	N/A	✅ 4354  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, receiver-1)	N/A	✅ 1845  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, other)	N/A	✅ 4355  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, receiver-1)	N/A	✅ 1845  ❌ 0  ⏭ 0    🔗	See Details
correctness-metrics	N/A	✅ 0  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, extension)	N/A	✅ 528  ❌ 0  ⏭ 0    🔗	See Details
integration-tests	N/A	✅ 59  ❌ 0  ⏭ 0    🔗	See Details

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