Allow serial number 0 for root CA certificates #6417

Job	Run time
build library (--enable-intelasm --enable-sp-asm)	36s
build library (--disable-asm)	34s
build library (--enable-intelasm --enable-sp-asm --enable-mlkem --enable-lms --enable-xmss --enab...	36s
build library (--enable-intelasm --enable-sp-asm --enable-all-crypto --enable-mlkem --enable-lms ...	55s
build library (--disable-asm --enable-all-crypto --enable-mlkem --enable-lms --enable-xmss --enab...	49s
build library (--disable-asm --enable-mlkem --enable-lms --enable-xmss --enable-mldsa)	32s
	4m 2s

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