mjcf parser: Fix unknown size vector parsing #165

Job	Run time
ubuntu-latest - Env all Release	40m 7s
ubuntu-latest - Env all Debug	3m 5s
ubuntu-latest - Env all-py39 Release	1m 22s
ubuntu-latest - Env all-py39 Debug	39m 57s
macos-latest - Env all Release	2m 2s
macos-latest - Env all Debug	2m 47s
macos-latest - Env all-py39 Release	22m 43s
macos-latest - Env all-py39 Debug	17m 35s
macos-13 - Env all Release	3m 15s
macos-13 - Env all Debug	40m 56s
macos-13 - Env all-py39 Release	45m 46s
macos-13 - Env all-py39 Debug	4m 34s
windows-latest - Env all Release	1h 11m 19s
windows-latest - Env all-clang-cl Release	3m 56s
check-macos-linux-windows-pixi	4s
	4h 59m 28s

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