In most cases, this means updating Subject subclasses and their assertThat methods to accept null actual values. (They should always accept null so that assertions like "assertThat(foo).isNull()" succeed instead of throwing NullPointerException.)
Occasionally, it involves other changes, like writing `isGreaterThan(1L)` instead of `isGreaterThan(1)` to resolve an ambiguity it Kotlin overload resolution.
PiperOrigin-RevId: 511776581
