Why does takeIf allow nullable receivers in its predicate?

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The signature of takeIf (and takeUnless) is:

public inline fun <T> T.takeIf(predicate: (T) -> Boolean): T?

instead of:

public inline fun <T : Any> T?.takeIf(predicate: (T) -> Boolean): T?

Why does Kotlin allow null as a possible value passed to the predicate? What is the rationale for not restricting T to non-null types?