Fix LightmapGI causes crash when using `--headless` #36196

Job	Run time
📊 Static checks / Code style, file formatting, and docs	32s
🐧 Linux / Editor w/ Mono (target=editor)	5m 46s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	38m 10s
🍏 iOS / Template (target=template_release)	1m 37s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	8m 10s
🍎 macOS / Editor (target=editor, tests=yes)	27m 38s
🤖 Android / Editor (target=editor)	4m 39s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	7m 44s
🍎 macOS / Template (target=template_release)	22m 20s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	3m 40s
🐧 Linux / Template w/ Mono (target=template_release)	6m 31s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	3m 49s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	6m 37s
🐧 Linux / Minimal template (target=template_release, everything disabled)	3m 21s
🌐 Web / Template w/o threads (target=template_release, threads=no)	7m 1s
🏁 Windows / Editor (target=editor, tests=yes)	10m 35s
🏁 Windows / Template (target=template_release)	9m 59s
🪲 Godot CPP / Build and test Godot CPP	7m 56s
	2h 56m 5s

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