fix(es/minifier): Check variable type while optimizing +=

[kdy1]

Description:

Related issue:

• Closes Concatenation of += operator #9504

[changeset-bot]

⚠️ No Changeset found

Latest commit: 4d31b74

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[codspeed-hq]

CodSpeed Performance Report

Merging #9575 will not alter performance

_{Comparing kdy1:issue-9504 (4d31b74) with main (6d15d9c)}

Summary

✅ 178 untouched benchmarks