Nullness improvements for simple core transforms - #40022
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TimestampedValue is immutable, so a TimestampedValue<V> is safe to use wherever a TimestampedValue of a supertype of V is expected. Saying so lets the nullness checker accept passing a TimestampedValue<T> where a TimestampedValue<@nullable T> is wanted, which combiners need when their accumulator starts out holding a null value. This only loosens checking, so it cannot invalidate existing callers.
Replace the blanket class-level nullness suppression with narrow, documented suppressions on the methods that genuinely produce a null. LatestFn's accumulator holds a null value until the first input arrives, so it is now typed TimestampedValue<@nullable T>. That is invisible in the public API, since Latest.combineFn() erases AccumT to a wildcard. It relies on TimestampedValue being covariant in its value type, so that an input can still be passed where an accumulator is expected. No runtime changes beyond hoisting the timestamp comparison into a private helper so that mergeAccumulators no longer routes through addInput. The null-element check it relied on is retained.
Remove the blanket class-level nullness suppression. The representative type of WithRepresentativeValues is genuinely optional -- expand() already branches on it -- so annotate it rather than suppressing. No runtime changes.
Remove the blanket class-level nullness suppression. Reifying a view in the global window creates a single placeholder element, which is a null Void; spell that type out rather than casting. No runtime changes.
Remove the blanket class-level nullness suppression, keeping the unrelated rawtypes one. AnyValueCombineFn.extractOutput returns null when the input is empty, and OutputT is T in the public signature of anyValueCombineFn(), so that keeps a narrow suppression. No runtime changes.
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Remove class-level blanket suppressions with targeted suppressions and code improvements.
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