Revert "base64ct: reject zero-length decode requests (#1387)" (#1714) #2318

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	58s
fuzz	2m 11s
build (1.81.0, wasm32-unknown-unknown)	55s
minimal-versions	0s
build (stable, thumbv7em-none-eabi)	45s
build (stable, wasm32-unknown-unknown)	47s
test (1.81.0)	2m 31s
test (stable)	2m 21s
	10m 28s

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