Prevent `encode_variant` doing `memcpy` from `nullptr` #14629

Job	Run time
📊 Static checks / Code style, file formatting, and docs	51s
🍎 macOS / Editor (target=editor, tests=yes)	4m 54s
🐧 Linux / Editor w/ Mono (target=editor)	5m 13s
🍎 macOS / Template (target=template_release)	4m 12s
🐧 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)	18m 46s
🍏 iOS / Template (target=template_release)	3m 4s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	10m 28s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	8m 39s
🌐 Web / Template (target=template_release)	1m 47s
🏁 Windows / Editor (target=editor, tests=yes)	10m 58s
🐧 Linux / Template w/ Mono (target=template_release)	2m 25s
🤖 Android / Template (target=template_release)	4m 56s
🏁 Windows / Template (target=template_release)	9m 24s
🐧 Linux / Minimal template (target=template_release, everything disabled)	52s
🪲 Godot CPP / Build and test Godot CPP	12m 7s
	1h 38m 36s

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