url: fix URLSearchParams(null) per spec #7721

Job	Run time
Build slim tarball	1m 17s
aarch64-linux: Cache V8 build	23s
x86_64-darwin: with shared libraries	32m 28s
x86_64-linux: with shared libraries	24m 14s
aarch64-darwin: with shared libraries	25m 24s
aarch64-linux: with shared boringssl-0.20260508.0	22m 32s
aarch64-linux: with shared openssl-1.1.1w	22m 30s
aarch64-linux: with shared openssl-3.5.6	23m 36s
aarch64-linux: with shared openssl-3.0.20	22m 59s
aarch64-linux: with shared openssl-3.6.2	23m 31s
aarch64-linux: with shared openssl-4.0.0	23m 57s
	3h 42m 51s

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