Explicitly null check the stdlib #642

Job	Run time
scala-library-nonbootstrapped	2m 32s
scala3-library-nonbootstrapped	2m 35s
tasty-core-nonbootstrapped	2m 36s
scala3-compiler-nonbootstrapped	4m 43s
test-tasty-core-nonbootstrapped	2m 53s
scala3-sbt-bridge-nonbootstrapped	4m 50s
scala-library-bootstrapped	5m 21s
scala-library-sjs	5m 24s
test-scala3-sbt-bridge-nonbootstrapped	5m 0s
scala3-library-bootstrapped	0s
scala3-compiler-bootstrapped	0s
tasty-core-bootstrapped	0s
scala3-tasty-inspector	0s
test-tasty-core-bootstrapped	0s
scala3-staging	0s
test-scala3-sbt-bridge-bootstrapped	0s
scala3-sbt-bridge-bootstrapped	0s
scaladoc	0s
scala3-library-sjs	0s
	35m 54s

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